Dr Piotr Wozniak, SuperMemo R&D Prof. Edward Gorzelanczyk, Dept. of Neuropsychology and Behavioral Genetics, University of Bydgoszcz Summer 2003

After publishing Good sleep, good learning, good life , we have received lots of mail with requests to clarify some of the complexities of sleep theory. Many people have sleep problems and look for simple solutions applicable in their lives. The present article attempts to explain the main reason for which healthy people may not be getting refreshing sleep: sleep phase problems . The article is accompanied by a simple freeware SleepChart application that may help you plan optimum sleep.

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The cycle of sleep and waking is regulated by the body clock. Body clock is located in the brain (suprachiasmatic nucleus). The clock has a period of about 24 hours. During a single 24 hour day we have a period of 6-10 hours when we are very sleepy. This is the time when we normally sleep. During the remaining 14-18 hours we are usually awake; however, only a portion of that waking time is suitable for intellectual effort. This period of maximum alertness may last as little as 2-6 hours. We should plan our day in such a way that sleep comes at the time of maximum sleepiness, while activities that demand maximum focus or creativity fall into the hours of maximum alertness. It is very difficult and usually very unhealthy to force your body and your body clock to do what you wish. It is far easier to do the opposite: adapt your life to your body clock. This article is intended to help you understand your body clock.

Delayed Sleep Phase Syndrome

Delayed Sleep Phase Syndrome (DSPS) is a sleep disorder in which an individual finds it difficult to fall asleep late in the night, and sleeps well into the afternoon if not awakened. DSPS has only been characterized in 1982, but increasing data indicates that various degrees of DSPS occur with epidemic frequency, esp. among high school and university students. DSPS individuals often like to keep on learning late into the night, go to sleep very late (for example, 4-6 am), and find it very hard to wake up early on a regular basis. For example, regular getting up at 7 am is a pure torture for individuals affected with DSPS. They often fail to keep jobs that require them to perform early in the morning. Very often, they tend to split the day sleep into two components. For example, DSPS students often get a short sleep in the night, wake up early with an alarm clock, go to school where they are semi-conscious and perform poorly, get a solid nap after school and only late in the evening they regain vigor and their full mental powers. DSPS students feel best after midnight when everyone else is asleep and they can focus on learning or other activities (reading, Internet, watching TV, computer games, etc.).

The main factors contributing to DSPS:

increased period of the body clock (well above 25 hours)

reduced or increased sensitivity to factors that reset or advance body clock (e.g. light, activity, stress, etc.)

electric lighting, 24-hour economy and the resulting "want to do more" lifestyle

A normal individual has a body clock running with a period slightly longer than 24 hours. The clock is reset in the morning with activity and bright light. Thus a normal individual easily adjusts to the standard day-night cycle. However, DSPS individuals may have their clocks running periods long enough to find it hard to fit to 24 hours. They also push their clocks ahead by activity late in the evening (the process opposite to the morning reset synchronization). DSPS individuals, when given a chance to sleep when they want, will tend to go to sleep later and later. They will also wake up later and later. DSPS people do not have problems with sleep if they sleep in their favorite hours. Most mild DSPS cases can be remedied by changes in lifestyle, but rarely are those changes painless to individuals affected with the condition.

If this description fits your problem, you may diagnose the degree of your DSPS with SleepChart freeware at the bottom of this article.

Advanced Sleep Phase Syndrome

Advanced Sleep Phase Syndrome (ASPS) is the opposite of DSPS. People suffering from ASPS get very sleepy early in the evening and wake up very early in the night. They circadian clock runs at less than 24 hour period. As they constantly struggle to survive to a reasonable evening hour, they sleep less, awake early and experience increased tiredness during the day. While a typical DSPS person is an adolescent student, a typical ASPS person is a retiree or a middle age low stress tolerance woman. The link between the age and sleep phase disorders may be related to aging itself; however, it may also be a result of lifestyle changes that come with age.

Correlates of sleep phase syndromes

DSPS is more prevalent among adolescents, while ASPS is more frequently observed in aging population. DSPS is also by far more frequent among students, programmers, avid readers, passionate artists, computer game addicts, etc. ASPS seems more likely in individuals whose life is deprived of intense stimulation (esp. in the evening), who meet fewer new challenges, or who are not facing information overload, stress, etc.

Finally, there is a complex relationship between DSPS/ASPS and depression/mania. On one hand, there may be a link between DSPS and manic personalities. Anti-depressants tend to increase the period of the body clock (e.g. clorgyline, imipramine). On the other, paradoxically, DSPS individuals may be more likely to suffer from depression (e.g. when in conditions of insomnia, sleep deprivation, resulting social problems, etc.). Similarly, low-stress tolerance depressed individuals are, more likely to suffer from ASPS. Again, when they are forced to adapt to "normal" life, their symptoms of depression tend to weaken as sleep deprivation here counteracts depression. The cause-effect relationship between sleep phase disorders and mood disorders is complex. Understanding it will contribute substantially to solving the escalating epidemic of sleep phase disorders.

It is not known which are the predominant underlying physiological factors that result in sleep phase disorders. Family clusters show that genes may affect the length of the circadian period. The lifestyle may affect levels of neurotransmitters and these could affect the period of the circadian clock. Conversely, the level of neurotransmitters may select for a specific lifestyle choices. Age may have a direct impact on the clock circuits, it may affect neurotransmitters or it can affect the lifestyle. Last but not least, sleep phase disorders will affect the mood and the levels of neurotransmitters in varying ways depending on whether free running sleep is used to remedy the disorder or wherever the individual attempts to fit a predetermined desirable sleep schedule.

How body regulates sleepiness

There are two main mechanisms that regulate sleepiness. One is the body clock and the other is the "wake-meter". Body clock produces increased sleepiness every 24 hours. The wake-meter increases sleepiness with prolonged wakefulness (i.e. the longer we do not sleep, the sleepier we are). In sleep literature, these two mechanisms are called the circadian and homeostatic regulation of sleep.

Sleep control components:

circadian clock - circadian clock produces sleepiness in 24 hour cycles

- circadian clock produces sleepiness in 24 hour cycles homeostatic control - wake-meter measures the period in which we stay awake and triggers sleepiness after we stay up for long enough

In DSPS or ASPS people, there may exist a combination of several factors that make it harder to get good sleep:

Circadian clock runs in periods far different from 24 hours. In DSPS people, the circadian clock may be set to 25-26 hours. In ASPS people, it could be 23 hours

Circadian clock is not sensitive to time resetting factors (professionally termed zeitgebers ). Normal people reset their clock in the morning by light and activity. In addition, darkness and inactivity in the evening provide further clues for the clock. Normal people with normal lifestyles can easily synchronize their sleep with the day-night cycle

). Normal people reset their clock in the morning by light and activity. In addition, darkness and inactivity in the evening provide further clues for the clock. Normal people with normal lifestyles can easily synchronize their sleep with the day-night cycle Homeostatic wake-meter shows changed sensitivity. Sensitive wake-meter will make people get tired very quickly after awakening. Insensitive wake-meter may make people tend to stay up for long

Lifestyle has a dramatic effect on the behavior of the circadian and homeostatic sleep regulation mechanisms. The same individual will show different sleep rhythm depending on such factors as: using artificial lighting, exercise, level of stress, timing of exciting activities, napping, diet, climate, changes in ambient temperature, health status, etc.

Examples of different sleeping rhythms

Freeware SleepChart application that comes with this article (bottom) makes it possible to take a snapshot of sleep that graphically illustrates DSPS and ASPS rhythms.

24 hour cycle. Let us first consider an ideally synchronized 24-hour cycle. In the picture below, an octogenarian female wakes up naturally everyday around 3:00-3:30 am. She sleeps 5.4-5.5 hours per day, wakes up refreshed and is active throughout the day. There is no synchronization with daylight as the waking hour falls into the period of darkness. The cycle is synchronized by evening activities. The subjects keeps in her mind a "must go to sleep" hour estimation that helps synchronize body clock with the time of day. This "psychological imprint" is illustrated by a smooth change in the sleeping rhythm after the end of the daylight saving time on Sunday October 27, 2002 (the graph disregards DST so that the waking hour before the change is set at 2:00-2:30 am).

24 hour cycle and stress. Let us now consider the impact of stress on a well-synchronized sleeping rhythm. In the picture below, a middle-aged self-employed male wakes up naturally everyday around 6:00-6:20. However, on June 3, 2003, a severe family problem throws the rhythm into chaos as evidenced by frequent nocturnal awakenings. The rhythm returned to norm one month later as soon as the family conflict was resolved.

Asynchronous DSPS. People suffering from DSPS find it difficult to synchronize with 24-hour clock. In the picture below, adolescent female with a mild DSPS suffers disintegration of the sleeping rhythm due to the failed efforts to synchronize with "civilized" life. After the vacation period, she begins in early September well-synchronized with the "rest of the world". She wakes up between 6:30 and 10:00. However, her body clock experiences continuous shift of optimum sleeping hours. Soon she wakes up at 12:00 and begins a struggle against the further shift. This results in disintegration of the sleep cycle, short sleep periods (e.g. 4 hours) and frequent bouts of tiredness. The presented software attempts to plot the hours of maximum natural sleepiness (between red and brown lines). Clearly, the greatest disturbance in sleep occurs at the point where the "natural" rhythm departs furthest from the "desired" rhythm. Mild DSPS cases are able to force the body clock to remain more or less in the desired bounds at the cost of constant struggle with sleepiness. In more severe cases, the circadian variables will run a 24 hour cycle and the individual will experience return to "good sleep" when free running variables align again with the "desired" sleep period. In the graph below, October 7 sleep period is almost aligning with the hours of maximum sleepiness resulting in good long refreshing sleep. The average sleep length is 6.8 hours but it changes widely. The average DSPS shift is difficult to determine due to the "struggle" with the natural rhythm; however, if is likely to be above 60 minutes as evidenced at the beginning of the graph.

Synchronous DSPS. A graph below presents a sleep rhythm of a middle-aged self-employed male. A very clear and regular DSPS pattern visible in the graph with a daily phase shift of 64-68 minutes. Although sleeping in "unnatural" hours is certainly less beneficial healthwise than normal sleep, for a DSPS patient, free running sleep rhythm may by far less stressful and disruptive than any attempt to fit to "standard" lifestyle (as shown above). There have been cases in literature that documented people living along such shifting DSPS schedule for decades without major health side effects although some authors claim increased incidence of depression, alcoholism or dependence on sedatives (as a result of attempts to induce sleep). A very reliable determinant of synchronous DSPS is the disruption of the link between the sleep onset hour and sleep duration. As the duration of sleep is determined by the circadian phase, well-synchronized sleep schedule shows little variability in the sleep length (6.6 hours in the graph below). In particular, the sleep length is independent of the sleep onset hour. Wherever the patient makes any attempt to synchronize with daylight or daylight-related activities, the link between sleep length and the onset hour will be reconstituted. Mistakenly, DSPS people are often called "owls" for their tendency to stay up late, while ASPS people are called "larks". The graph below illustrates why this is a misnomer.

SleepChart: A sleep charting and scheduling application

You can download a freeware SleepChart application by clicking the link at the bottom of the article.

All you need to do in the program is to click the beginning and the end of the sleep block in the graph. See the bottom of the display for exact hour points corresponding to the position of the mouse pointer. If you set a wrong block, select it by clicking and press Del.

SleepChart will attempt to approximate the circadian low that correlates with high sleepiness, low temperature, low ACTH, high melatonin, etc. The underlying assumption is that you do not attempt to regulate your sleep. This means that you do not wake up with an alarm clock or do not struggle against the sleepiness in the evening (e.g. in order to delay sleep). Currently there is no option that would let you eliminate artificially-regulated sleep blocks. This means that all attempts to modify your sleep schedule will fool the algorithm and your reading will be inaccurate or plain wrong. It is also very important that you do not attempt to follow the circadian approximation when determining your optimum sleep hours! This can result in a positive feedback of error and disintegration of the sleep cycle. In other words, errors in the graph may be amplified by your attempts to follow the graph. At worst, you could even self-diagnose yourself with DSPS without actually suffering from the disorder! Your only and sole "go to sleep" criterion should be rapidly increasing sleepiness. You may use the graph to approximate the moment in which the readiness for sleep will occur. You can also find it helpful in chronotherapy for ASPS or DSPS to make it easier to schedule your appointments without conflicting with your natural sleep rhythm.

Circadian graph in DSPS

The circadian graph can help you better understand your sleep rhythm with its homeostatic and circadian components. You will need a few months of data before the graph becomes meaningful. In addition, sleep boundary approximation lines are subject to substantial hysteresis. If your lifestyle changes dramatically (e.g. as a result of therapy), you may need a few weeks for the approximation lines to align properly with data. The circadian graph may then be more difficult to interpret. Having separate data sets for such circumstances solves the problem.

Homeostatic: The blue line corresponds with homeostatic sleepiness derived from the number of sleep blocks falling into a given hour of the waking day (i.e. where zero refers to the hour of waking). It roughly expresses your "tiredness of wakefulness". It also expresses your ability to fall asleep. From the graph below you can read that the best chances of falling asleep fall around 18th hour of wakefulness (assuming the graph is created without any artificial form of regulating sleep such as the alarm clock, delaying sleep, etc.). Your own optimum hour is naturally likely to differ (check with SleepChart). For most people the optimum hour falls into the range 16-20. If your graph looks like below and you wake up at 6 am, you can conclude that you can best go to sleep shortly before midnight (so that you could ensure you are in bed when the 18th hour of wakefulness comes up). Some people take naps during the day. Others don't. The graph below is typical of a regular napper with the optimum nap time coming at 7-8 hour of wakefulness. For example, if you get up at 6 and your optimum nap time hits the 8th hour, you should take a break around 14 and look for a secluded place for a few minutes of rest. Naturally, planning your lunch at 13:00 or so would be recommended too. In the graph below, 8.2% of sleep blocks begin in the 18th hour (night sleep), while another 8.1% begin in the 8th hour (nap). Non-nappers will also experience a peak of sleepiness around the 7th hour even though their blue-line graph will not show as prominent a bulge as below.

Circadian: The red line corresponds with circadian sleepiness derived from the average length of sleep blocks started at a given hour of the waking day. The graph indicates that even if you are able to initiate sleep during the day, it will never last long. Only after 14 hours of waking does the length of initiated sleep reach beyond 6 hours. Maximum length of sleep can be achieved at 15th hour; however, this does not indicate this is the optimum hour of going to sleep. If sleep is initiated too early, it may or may not make it possible to catch on the full circadian low. In other words, we can wake up after just a couple of minutes of sleep and make it harder to fall asleep again (due to instantly losing the homeostatic sleepiness). In addition, sleep initiated before the full circadian low does not seem to be of more value that slightly shorter sleep initiated a bit later (e.g. as reflected by the subjective feeling of being refreshed in the morning, or as measured polysomnographically). The blue homeostatic line indicates that the sleep is more likely initiated at 18th hour, while its average length is 6.5 hours. If your graph is generated without attempts to artificially regulate sleep, the second peak in the homeostatic curve may indicate your optimum hour of retirement. The graph indicates that if you delay your sleep by one hour, it will be shortened by 10-30 minutes. It is possible, that even this little shortening will affect your performance during the day. If you advance your sleep by one hour, it may be 10-30 minutes longer but its quality is not likely to increase. In addition, you run the risk of premature awakening (breaks in sleep are ignored in the graph as this makes it easier to determine the circadian low).

Phase shift graph in DSPS

The phase shift graph may be used by people suffering from ASPS and DSPS. This graphs helps you see at what time you are likely to fall asleep for a given waking time. For example, in the graph below, you can see that if your waking time comes at 7 am, your best retirement hour (blue line) comes at 1 am. However, if you happen to wake up naturally at 11 am, you are not likely to be ready for sleep before 5 am the next day. The lighter pink and gray lines indicate the timing of a nap. Again, if you wake up at 7 am, your best napping time might occur between 15:00 and 16:00. Remember! Each individual will have a his or her own unique graph. Moreover, the graph will look differently if it is taken at times of work or at times of summer vacation. It will be affected by stresses at work and at home. It may even change when you move from one house to another, or when you change the climate zone. The graph will slope only for people with sleep phase problems and will accurately reflect your rhythm only if you adhere to free-running sleep. If you use an alarm clock, this graph will be meaningless!

Download: To download SleepChart, a freeware sleep scheduling application for Windows, click here

Note for users of SuperMemo: The algorithm used by SleepChart may be integrated with future versions of SuperMemo to provide an optional research and repetition scheduling tool that will help you optimize the timing of your learning, and better understand your circadian patterns that are vital for optimizing your sleep and learning. Over years, this tool will hopefully reveal intimate links between circadian phase, alertness, recall, memory consolidation, retention and forgetting. Your sleep data can provide invaluable contribution to this lofty long-term goal. Thank you in advance for your submissions.

Use sleep(AT)supermemo(.)com to send your sleep data for analysis (simply use the mail button in the SleepChart application). If you would like to see the source code of this application published, write to support