Jet lag arises from a misalignment of circadian biological timing with the timing of human activity, and is caused by rapid transmeridian travel. Jet lag's symptoms, such as depressed cognitive alertness, also arise from work and social schedules misaligned with the timing of the circadian clock. Using experimentally validated mathematical models, we develop a new methodology to find mathematically optimal schedules of light exposure and avoidance for rapidly re-entraining the human circadian system. In simulations, our schedules are found to significantly outperform other recently proposed schedules. Moreover, our schedules appear to be significantly more robust to both noise in light and to inter-individual variations in endogenous circadian period than other proposed schedules. By comparing the optimal schedules for thousands of different situations, and by using general mathematical arguments, we are also able to translate our findings into general principles of optimal circadian re-entrainment. These principles include: 1) a class of schedules where circadian amplitude is only slightly perturbed, optimal for dim light and for small shifts 2) another class of schedules where shifting occurs along the shortest path in phase-space, optimal for bright light and for large shifts 3) the determination that short light pulses are less effective than sustained light if the goal is to re-entrain quickly, and 4) the determination that length of daytime should be significantly shorter when delaying the clock than when advancing it.

When our body's internal timekeeping system becomes misaligned with the time of day in the outside world, many negative effects can be felt, including decreased performance, improper sleep, and jet lag. When misalignment is prolonged, it can also lead to serious medical conditions, including cancer, cardiovascular disease, and possibly even late-onset diabetes. Rapid readjustment of our internal daily (circadian) clock by properly timed exposure to light, which is the strongest signal to our internal circadian clock, is therefore important to the large proportion of the population which suffers from misalignment, including transmeridian travelers, shift workers, and individuals with circadian disorders. Here we develop a methodology to determine schedules of light exposure which may shift the human circadian clock in the minimum time. By calculating thousands of schedules, we show how the human circadian pacemaker is predicted to be capable of shifting much more rapidly than previously thought, simply by adjusting the timing of the beginning and end of each day. Schedules are summarized into general principles of optimal shifting, which can be applied without knowledge of the schedules themselves.

Using this approach, we determined over 1,000 schedules that optimally re-entrain, without the limits on the length of schedules imposed by prior studies. Moreover, while previous work often assumed that the light available for shifting was 10,000 lux or higher, we considered many light levels, including those found indoors.

Here we describe a mathematically robust method, which uses existing mathematical models [16] , [17] , without any simplifying assumptions, to produce schedules that are locally optimal. These schedules are proven to outperform any other schedules which are not locally optimal. (A detailed statement of these methods is provided in supplemental text S1 ) We also show how these schedules and this methodology can be applied to the more subtle problem of partial re-entrainment (See Designing Schedules for Partial Re-entrainment in supplemental text S1 ). Our method is fast, accurate, and broadly applicable to a wide variety of problems of biological oscillation. It avoids the difficulties encountered in prior studies by accomplishing the following: 1) It requires as a constraint that the final phase be exactly entrained, but allows, using a penalty, other variables (including circadian amplitude) to deviate from their average values within experimentally observed ranges. 2) It solves some equations forward in time and other equations backwards in time. 3) It recognizes that the best light schedules are bang-bang (terms which are bold/underlined are defined in the glossary, supplemental text S2 ), and consist of periods of either darkness or maximum light levels, and 4) It uses the slam shift as a starting point for the optimization. This last part of our methodology is not an assumption, but a property of optimal schedules. We justify this usage in the Methods section.

Mathematical models can, in theory, be analyzed to determine optimal schedules, or schedules which outperform all others [29] . In practice, however, this analysis is quite difficult. For this reason, previous studies have used mathematical simplifications, which unfortunately severely restrict the types of schedules considered. These prior studies have light exposures of a fixed intensity and duration [9] , ignore the dynamics of phototransduction [30] , [31] , optimize each time-point separately [30] , [32] , [33] , rather than considering how to optimize the whole schedule together, minimize light levels, rather than minimizing time to entrainment [34] , [35] , and consider only schedules where circadian amplitude is relatively unperturbed [9] . The problem with these prior studies is that such simplifications have been shown to yield suboptimal schedules, which can result in nearly double the amount of time needed to re-entrain when compared with optimal schedules [31] .

Accurate mathematical models for the effect of light on the human circadian system are available [16] , [17] . Several studies have demonstrated their value in practical applications. Specifically they have been used to design experimental protocols i.e. the so-called “forced desynchony” protocol [18] ; their predictions have been validated against experimental data [19] ; and they have been used effectively in numerous real-world applications [20] . These models contain two components: “Process L,” which simulates phototransduction in the retina [21] , and a “Process P,” whose two variables represent the state of the central circadian pacemaker, located in the suprachiasmatic nucleus of the hypothalamus. The choice of a “Process P” model with at least two variables is important, because it represents both circadian phase and amplitude [22] . More detailed biochemical models are available [23] , but these can be reduced to models similar to [16] and [17] , and have yet to be fit to human data, making them a less attractive choice. Additionally, many features of the biochemical models, such as a narrow range of entrainment [24] or the presence of a sharp threshold separating orthodromic (the same direction as the schedule shift) and antidromic (the opposite direction) re-entrainment [25] , are captured by models of this type. The process of re-entrainment involves multiple oscillators (i.e. in tissues of the body [26] and in regions of the SCN [27] ), the dynamics of which are not captured by a single-oscillator model. Multiple-oscillator models are available i.e. [28] , however while such models are very promising for studying re-entrainment, they too have not yet been fit to human PRC data and are far less widely used than [16] and [17] . This makes them less attractive, at least until their parameters are fit to human data.

Light is the strongest signal to the human circadian system [6] . Light can slow (phase delay), or speed (phase advance) the circadian clock, depending on when it is administered. When properly timed, light can reduce the amplitude of the circadian clock, making it more sensitive to subsequent light signals [7] . Previous light history has been shown to affect re-entrainment as well, and may also increase or decrease sensitivity to light [8] . A large literature exists offering suggestions on how to time light exposure to quicken re-entrainment and avoid jet lag. Suggestions include additional pulses of light [9] , amplitude suppression [10] , intermittent light [11] , [12] , and avoidance of morning light [13] . These suggestions are important, since it has been shown that the choice of light schedule can significantly quicken [14] or slow [15] re-entrainment. Given the infinite variety of possible schedules, however, it is unrealistic to attempt to find schedules that re-entrain in the minimum amount of time through experimental methods alone.

Modern society requires individuals to be awake and alert at times that conflict with their internal circadian (∼24-hour) timekeeping systems. In year 2012 over 60 million Americans traveled overseas, subjecting themselves to long periods of circadian mistiming, impaired sleep, and low performance [1] . Over one fifth of American workers follow non-standard schedules, which place them at increased risk for sleep-related accidents [2] , [3] . Circadian misalignment has also been linked to many health problems [4] , [5] . Thus, after a schedule shift, it is important for individuals to reach a state of proper circadian timing (entrainment) as quickly as possible, and to minimize the time spent between entrained states (re-entrainment).

Results

Each optimal schedule for complete re-entrainment gives the pattern of light and dark (LD) which will entrain the existing model [16] in the minimum time (See supplemental text S1). Each optimal schedule for partial re-entrainment gives the LD pattern which will place CBTmin at the start of the SD region in the minimum time (See Designing Schedules for Partial Re-entrainment in supplemental text S1). Optimal schedules for partial re-entrainment can be derived from those for complete re-entrainment. Both types of schedules consist of only two light levels—one as dim as possible and the other as bright as possible. This is not an assumption, rather, we demonstrate that light should always be at its minimum or maximum level if the goal is to entrain quickly. Moreover, each 24-hour phase of the schedule consists of one day phase and one night phase. Thus to shift optimally, a traveler needs only to change the timing of his or her dawn and dusk. This is both practical and somewhat surprising, since we find these optimal schedules to outperform many others, including schedules which are extremely difficult to follow (i.e. those with continuously fluctuating light levels, or with multiple light/dark phases i.e. an LDLD cycle). It is also encouraging that the optimality of long light exposures agrees with previous studies, which have asserted that while brief pulses of light may be much more effective per photon of light, continuous light still provides the most drive to the circadian system [21].

In figure 1, we compare our optimal schedules (1F and 1G) to five other previously proposed schedules (1A–1E) for re-entrainment to a 12-hour time-zone shift (1A). We present all seven schedules as actograms, where each new line represents a subsequent day. Black indicates darkness. Gray indicates dim light (5 lux). White indicates low room light (100 lux). Yellow indicates bright light (10,000 lux). In the original time zone, days −4 to −1 show a light-dark schedule of 16 hours of 100 lux light and 8 hours of darkness (LD 16∶8). At time 0 of day 0, we assume a transition occurs. A magenta triangle shows the predicted timing of the core body temperature minimum (CBTmin), a key circadian marker that, when entrained, occurs slightly after the midpoint of the dark episode. The brightness of the triangle's face represents the strength of the timekeeping signal, or the circadian amplitude, with white corresponding to zero amplitude. A blue dot also shows the predicted timing of CBTmin, except under conditions approximating real-world variations in light-levels and inter-individual differences, the details of which are explained in the sequel. The blue dots predict the CBTmin of twenty hypothetical subjects, rather than one.

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larger image TIFF original image Download: Figure 1. Comparison of schedules for a 12-hour shift of the light-dark cycle. Predicted circadian phase, indicated by simulated core body temperature minima (CBTmin, magenta triangles), is plotted against the pattern of exposure to bright light (10,000 lux, yellow), moderate light (100 lux, white), dim light (5 lux, gray), and darkness (0 lux, black). Predicted CBTmin under noisy light levels (See supplemental figure S1), with circadian period randomly sampled from an experimentally measured distribution [18], is plotted for 20 hypothetical subjects (blue circles). Circadian amplitude at CBTmin is indicated by the brightness of the markers, with white corresponding to zero amplitude. The timing of entrained CBTmin in the new time zone is indicated by the dotted line. The subjects are initially entrained to a 16∶8 LD-cycle in moderate light. At day 0 the schedule shift occurs. The six schedules are compared are: (A) The abruptly shifted LD-cycle; also called a slam shift. (B) Times of light exposure and avoidance in the new time zone are prescribed to quicken re-entrainment. Phase delays of 1 hour/day are assumed. Based on the recommendations proposed by R. Sack (See Supplementary Appendix of [49].) (C) Times of light exposure and avoidance are prescribed, with an assumed phase delay of 2 hours/day. Based on the recommendations proposed by J. Waterhouse et.al. (See Table 2 of [50].) (D) The sleep/dark region is gradually delayed, with 2 hours of bright light before bed and 2 hours of light avoidance after wake. Assumed delay of 2 hours/day. Based on the recommendations proposed by C. Eastman et.al. in [51]. (E) A PRC is used to place a series of 5 hour light stimuli, in a background of dim light, in order to produce a large delay. The timings are refined using a model [16]. Proposed by D. Dean et.al. (See Figure S1:E1 in [9].) (F) Our optimal schedule for complete re-entrainment in minimum time. A model [16] is used to compute the mathematically optimal schedule of light exposure (See Methods) which resets the model in the least possible amount of time. (G) Our optimal schedule for partial re-entrainment in minimum time, designed to place CBTmin at the beginning of the sleep/dark region as quickly as possible (See Designing schedules for Partial Reentrainment in supplemental text S1). https://doi.org/10.1371/journal.pcbi.1003523.g001

To study the effects of these schedules, we also plot the process of re-entrainment in terms of both phase and amplitude. Thus, we produce experimentally measurable predictions for phase-amplitude resetting maps (PARMs) [36] in figure 2. PARMs plot phase and amplitude simultaneously – the angle to the origin corresponds to the phase, and the radius corresponds to the limit-cycle amplitude. These maps show clearly how the optimal schedules – both for complete re-entrainment (1F) and partial re-entrainment (1G) – shift the circadian pacemaker along the shortest and straightest paths in phase-space (the space of phase-amplitude pairs) (See figures 2F and 2G). The optimal schedules involve partial circadian amplitude suppression, which occurs when light exposure is centered near the crossover between advancing and delaying regions of the day [37], and can also be observed between advancing and delaying schedules [38]. The existence of this mode of re-entrainment, and of circadian amplitude suppression in general, has been shown experimentally [7], [36]. Moreover, amplitude suppression has been found to be consistent across multiple circadian variables [39].

Our proposed schedule for optimal (minimum-time) complete re-entrainment to a 12-hour time zone shift takes approximately 4 days, whereas the five previously proposed schedules require more than 7 (see Table 1). Moreover, several previously proposed schedules require more than 13 days to achieve complete re-entrainment. Our schedule for optimal partial re-entrainment takes only 2 days, while other previously proposed schedules take 3 days or more. These large gaps in predicted performance between optimal schedules and those previously suggested strongly imply that current methods have large room for improvement.

Since real-world light levels are highly variable, we added variability in the lighting conditions to mimic the natural environment (See supplemental figure S1) [40]. We also allowed the period of the circadian pacemaker to be drawn at random from a distribution matching the human population [18]. We then re-simulated each schedule twenty times and plotted the predicted CBTmins with blue dots (figure 1). With the exception of figure 1B, which took more than ten days to achieve complete re-entrainment, we found that the other four schedules (1A, 1C, 1D, and 1E) were highly variable, while our optimal schedule (1F) had very little variability. Moreover, our optimal schedule for partial re-entrainment (1G) had most of its variability confined to the SD region, which still allows for good sleep. This is remarkable, since the optimal schedules we propose re-entrain in the minimum time, and are therefore not specifically optimized for robustness or noise. Thus we see that robustness is a feature of optimal, rapidly-shifting schedules.

We next computed optimal schedules for complete re-entrainment in minimum time to an 8 hour advance and an 8 hour delay (See figure 3). Again, re-entrainment was possible within three to four days when 10,000 lux of light was available, and our schedules were more reliable than those previously proposed (e.g. the so-called “slam shift”, in which the light/dark cycle is suddenly shifted to the new time zone). We did find antidromic re-entrainment (delaying when advancing should occur, a experimentally observed phenomenon [41]) in a small fraction of the simulated population when advancing by 8 hours (See 3C). We also found that to delay was easier than to advance (re-entrainment in two versus three days), matching conventional wisdom. The 8 hour advancing and delaying schedules also required less amplitude suppression than the 12 hour phase shifting schedule.

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larger image TIFF original image Download: Figure 3. Optimal schedules for re-entrainment to 8 and 12 hour shifts of the LD-cycle. Predicted core body temperature minima (CBTmin, magenta triangles) are plotted against the pattern of optimal exposure to bright light (200 lux–10,000 lux, yellow), moderate light (100 lux, white), and darkness (0 lux, black). Predicted CBTmin under noisy light levels (See supplemental figure S1), with circadian period randomly sampled from an experimentally measured distribution [18], is plotted for 20 hypothetical subjects (blue circles). The timing of entrained CBTmin in the new time zone is indicated by the dotted line. Circadian amplitude at CBTmin is indicated by the brightness of the markers, with white corresponding to zero amplitude. The subjects are initially entrained to a 16∶8 LD-cycle in moderate light. At day 0 the schedule shift occurs. Optimal schedules are grouped in rows by maximum admissible bright light level (yellow), and in columns by effected phase shift. Figures (A), (B), (C) use a maximum light level of 10,000 lux; (D), (E), (F) use 1000 lux; (G), (H), (I) use 500 lux; (J), (K), (L) use 200 lux; (M), (N), (O) use 100 lux. Figures (A), (D), (G), (J), (M) are optimal schedules for an 8-hour delay; (B), (E), (H), (K), (N) a 12-hour shift; (C), (F), (I), (L), (O) an 8-hour advance. https://doi.org/10.1371/journal.pcbi.1003523.g003

Reducing the maximum available light level ten-fold to 1,000 lux yielded optimal schedules similar to the schedules for 10,000 lux, except that complete re-entrainment required an additional day. Reducing the maximum light level an additional two-fold to 500 lux added another day to each schedule. While the daily light exposure in schedules for the 12 hour shift and 8 hour advance changed little with reduced light, the 8 hour delay showed very little amplitude suppression (3G) compared to when more light was available (3A and 3D). Reducing the maximal light level to 200 lux required an additional two to three days to shift. At this lower light level, no amplitude suppression occurred for the 8 hour advance or the 8 hour delay. When the maximum available light was further reduced to 100 lux, no amplitude suppression occurred for any shift. These transitions are clearly visible in the corresponding PARMs (See supplemental figure S2).

We next found optimal schedules for complete re-entrainment to all phase shifts with maximum light levels of 10,000, 1,000, 500, 200 and 100 lux consisting of approximately 1,000 optimized schedules. These are summarized in figure 4. Since schedules for partial re-entrainment in minimum time can be derived from schedules for complete re-entrainment (See Designing Schedules for Partial Re-entrainment in supplemental text S1), figure 4 can be used to find all optimal schedules for both complete and partial re-entrainment in minimum time.

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larger image TIFF original image Download: Figure 4. Optimal schedules for all phase shifts. This plot shows the pattern of bright light (200 lux–10,000 lux, yellow), background light (100 lux, white), and darkness (0 lux, black) under which the clock is optimally reset from the corresponding initial phase. If a vertical line is drawn on the plot, then the pattern of light and dark along this vertical line is the optimal schedule for resetting the clock from the corresponding initial phase. Figure (A) shows the optimal schedules for resetting the clock when 10,000 lux light is available; (B) 1000 lux; (C) 500 lux; (D) 200 lux; (E) 100 lux. https://doi.org/10.1371/journal.pcbi.1003523.g004

The format used in figure 4 was described by Winfree in section 1D of [22]. It is interpreted in the following way. Suppose we were to draw a vertical line on figure 4A at initial phase 8. Each point on this vertical line would correspond to a time on the vertical axis. If we were to follow this line down from day −2 to day 13, we would find that this line passes through a white region (100 lux) for 8 hours, then a black region (0 lux) for 8 hours, then white for 16, black for 8, white for 8. Then it crosses time , and passes through black for 8.9 hours, yellow (10,000 lux) for 7.8, black for 16.5, yellow for 8.1, and so on. This pattern of light and dark is the optimal schedule for re-entrainment to an 8-hour delay in 10,000 lux light – it corresponds exactly to days −2 to 13 of figure 3A (the optimal schedule itself only occupies days 0 to 3.5). Similarly, if we were to draw a vertical line on figure 4A at initial phase 12 hours and read off the schedule in the same way, we would get the optimal schedule for a 12-hour shift (figure 3B). A vertical line on figure 4A at initial phase 16 would likewise give the optimal schedule for an 8-hour advance (figure 3C).

The meaning of “initial phase” measured on the horizontal axis comes from the following interpretation. Without loss of generality, we fix our destination time zone, and consider shifts from all possible time zones. Each vertical slice corresponds to an optimal schedule for complete re-entrainment from the initial phase (where the slice intersects the horizontal axis) to phase zero. Days −2 and −1 show a 16∶8 light-dark (LD) cycle of 100 lux (dim home or office lighting) in the original time zone. At the beginning of day 0, the transition to the new time-zone begins. An optimal schedule for re-entrainment is presented, then once it is finished, the LD cycle in the target time zone takes over. Again, here black corresponds to darkness, white to dim light (100 lux), and yellow to bright light (200–10,000 lux depending on the intensity). We also simulate the predicted circadian response to these schedules, displaying both circadian phase and amplitude in figure 5. The format is identical to figure 4. Days −2 and −1 show the entrained state and thus may be used as a legend, associating a unique hue to each phase of the oscillator. After the transition occurs at day 0, the predicted state of the circadian pacemaker is shown, using hue to represent phase and brightness to represent amplitude. (The precise coloring, based on the state of the model variables, is shown in supplemental figure S3.) These schedules can be compared with the phase shifting predicted for a slam shift (See supplemental figure S4) which, particularly for lower light levels, does not phase shift the clock in the 12 days reported. Thus our optimal schedules are much more efficient than the slam shift.

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larger image TIFF original image Download: Figure 5. Predicted optimal circadian phase and amplitude for all phase shifts. The format is exactly the same as figure 4. Days −2 and −1 may be used as a legend associating a unique hue to each phase of the oscillator. Brightness is then used to represent amplitude, with white corresponding to zero amplitude [37]. The exact coloring, based on the state of the model variables, is shown in supplemental figure S3. Figure (A) shows the predicted phase and amplitude under the optimal schedules for resetting the clock when 10,000 lux light is available; (B) 1000 lux; (C) 500 lux; (D) 200 lux; (E) 100 lux. https://doi.org/10.1371/journal.pcbi.1003523.g005

From this plot (figure 5) and the PARMs (supplemental figure S2), we see that schedules can be separated into two classes. First, for 10,000 lux of light, or large (>8 hour) phase shifts with 1000 or 500 lux, the optimal schedules pursue the most direct path between the original state of the model and the re-entrained state. Thus in the PARMs the optimal schedule forms a nearly straight line between the state of the clock before the shift and the state after the shift. We call this pattern of optimal shifting minimum path shifting (MPS). For larger phase shifts with MPS (e.g. 8–12 hour time zone changes), the amplitude of the circadian pacemaker can be partially reduced in the middle of the schedule.

A second class of schedules is seen when the maximal light levels are lower (e.g. 200 or 100 lux) or for smaller (<8 hour) phase shifts with 500 lux or 1000 lux. These optimal schedules often shift with minimal changes to the preferred circadian amplitude. We call this type of shifting limit-cycle shifting (LCS). In LCS, light stimuli are presented which maximally advance or delay the circadian clock while near (but not on) the limit cycle; the next day's light stimulus is the same as the previous day's except that is presented at the appropriate time considering the phase shift predicted to occur. This is supported by figure 6: we compute the phase response curves (PRCs) to every possible 24-hour light pulse, and find that the stimuli which cause the maximum advances or delays in a single day match the optimal LCS schedules almost exactly. This is also supported by figure 4E (100 lux), in which no amplitude suppression occurs and therefore only LCS schedules are present. In this figure, the envelope indicating the time to re-entrainment is almost exactly a triangle – in the sense that the time to re-entrainment increases linearly with the magnitude of the phase shift. Moreover, from the fact that its peak occurs at a phase of 13 hours and a time of 12.7 days, we can estimate that the optimal number of hours delayed per day by 100 lux light is 13/12.7 = 1.03 hours/day and the optimal hours advanced is 11/12.7 = 0.87 hours/day. Compare this to figure 6, where the optimal number of hours delayed by a single stimulus of 100 lux light is 1.09 hours/day and the optimal hours advanced is 0.84 hours/day.

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larger image TIFF original image Download: Figure 6. Optimal one-pulse stimuli to advance or delay the clock. We simulated the PRCs to all possible one-pulse stimuli for a variety of different light levels. For each light level, two stimuli were selected: the one producing the greatest advance and the one producing the greatest delay. The model was kept in total darkness before the stimulus was administered. Resulting phase shifts were measured using the concept of isochrons [22]. On the right the optimal advancing stimuli (top) and optimal delaying stimuli (bottom) are plotted for each light level. The bars indicate both the duration (bar length) and phase (midpoint) of the light stimuli relative to the timing of CBTmin (solid magenta vertical line). On the left the PRCs corresponding to each optimal stimulus length are plotted. On each PRC, the optimal phase maximizing the shift is marked by a circle (filled for advances and unfilled for delays). The PRCs corresponding to optimal advancing stimuli are drawn using solid lines, and ones corresponding to optimal delaying stimuli, using dashed lines. We found that, for low light levels and smaller shifts, the daily light exposures observed in the optimal schedules (See figures 4 and 5) matched the optimal one-pulse stimuli very closely. In particular, we find that the optimal advancing stimuli are much longer than the optimal delaying stimuli. https://doi.org/10.1371/journal.pcbi.1003523.g006

An important feature of LCS schedules is that the daily stimulus to delay the clock is much shorter than the stimulus to advance the clock. Moreover, there appears to be region of phases where no light appears in either stimulus. This goes against the predictions given by instantaneous phase response curves (iPRCs), which provide a simpler, amplitude-free model of circadian response [42]. This discrepancy arises from the fact that even in LCS schedules, a small amount of amplitude suppression still occurs. To validate the prediction that light pulses which maximally delay the circadian clock are significantly shorter than ones which maximally advance it, we look again to figure 6. We find that shorter pulses do in fact accentuate the delay regions of the PRCs, while longer pulses accentuate the advance regions.

Repeating these computations on the model described in [17] gave us almost the same results (See supplemental figures S5, S6, S7, S8, S9, S10, S11), the major difference being that the optimal schedules for the Simpler model [17] favor less amplitude suppression than for the Jewett-Forger-Kronauer model [16]. This is reasonable, since the dynamics of amplitude suppression and amplitude recovery differ greatly between the two models [43]. Specifically, in the equations for the oscillator, the Jewett-Forger-Kronauer model uses a seventh-order nonlinearity while the Simpler model uses only a cubic nonlinearity. The fact that two models which are both qualitatively and mathematically different gave similar answers suggests that we have found general principles of optimal shifting that aren't dependent on a specific model.

Finally, while we present the schedules in a form (i.e. figure 3) which could be applied to practical problems, there are several practical limitations which must be taken into consideration. Many of the theoretical schedules we propose require either very long sleep/dark phases or very long bright light phases, which may be difficult to reconcile with real-world obligations. This makes the use of brief pulses of bright light much more appealing [44], however as we have observed (e.g. figure 1) restricting ourselves to such a strategy may significantly prolong the time to re-entrain. One possibility is the use of low-transmission or red-blocking glasses [44] in conjunction with the use of a light visor [45]. While such a strategy may make our schedules much more practical, its effectiveness awaits experimental validation.