Study Subjects

Table 1. Table 1. Characteristics of the Blind Patients.

We studied 11 blind patients with no conscious perception of light (Table 1) and 6 normal men ranging in age from 20 to 25 years. With the exception of disturbed sleep in some of the blind patients, no subjects had medical or psychiatric disorders, as determined by history taking, physical examination, chest radiography, electrocardiography, psychological questionnaires, and biochemical and toxicologic screening tests. None had worked at night in the past three years. Their sleep histories were evaluated in a structured interview and a sleep-disorders questionnaire21 modified to assess sleep disorders in blind patients. The possibility that sleep apnea and nocturnal myoclonus were the basis of the patients' difficulty in sleeping was excluded by polysomnography. All the subjects were instructed to maintain regular sleep–wake schedules and to record their bedtimes and awakening times for at least two weeks before the studies. The protocol was approved by the Human Research Committee of Brigham and Women's Hospital, and all subjects gave written informed consent.

Ophthalmologic Testing

The blind patients underwent a complete neuro-ophthalmologic examination that included observation of their behavior. Pupillary reflexes to light were evaluated with the brightest light of an indirect ophthalmoscope and were examined with a slit lamp. Complete electroretinographic testing, including narrow-band filtering,22 was performed in all eight blind patients with eyes; only electroretinograms for which there were control recordings, in which opaque filters shielded the eyes from the white 30-Hz stimulus, are discussed here. Visual evoked potentials were tested in six of the eight blind patients with eyes.

Ambulatory Evaluations

The subjects' wrist activity and core body temperature were monitored on an ambulatory basis (PMS-8 Recorder, Vitalog, Redwood City, Calif.) for at least one week before hospital admission. The sleep–wake schedules during the studies represented an average of the clock times recorded in the weekly diaries and were verified by ambulatory monitoring.

Constant Routines

Two-day constant routines consisted of enforced wakefulness with the subject in a semirecumbent position, with daily intake of nutrition and electrolytes evenly distributed throughout the day and night. This procedure attenuates the physiologic responses evoked by periodic behavioral and environmental stimuli, such as sleeping, eating, changing posture, and changing the intensity of ambient light, thereby permitting assessment of the endogenous circadian cycles of body temperature and melatonin secretion.3,23

Evaluation of Photic Input with the Melatonin-Suppression Test

Figure 1. Figure 1. Melatonin-Suppression Test in a Normal Subject (Upper Panel) and a Blind Patient (Patient 2, Lower Panel). Plasma melatonin and temperature were measured repeatedly during a constant routine (hatched bars) and subsequent episodes of sleep (solid bars). The light intensity was less than 0.02 lux during the sleep episodes, 10 to 15 lux during the constant routine, and approximately 10,000 lux during 90 to 100 minutes of exposure to bright light (open columns) 22 to 23 hours after the initial temperature minimum (crosses). In both subjects, plasma melatonin concentrations decreased markedly in response to bright light and then increased after the return to dim light.

During a constant routine, the normal subjects and blind patients were exposed to bright light for 90 to 100 minutes, with the exposure timed to coincide with the expected peak in plasma melatonin concentrations (Figure 1). The midpoint of the bright-light exposure occurred 22 to 23 hours (mean [±SD], 22.6±0.4) after the fitted temperature minimum (as defined below under Statistical Analysis), which has a consistent phase relation with the fitted peak of the melatonin rhythm.24 The test results were defined as positive when the average plasma melatonin concentration during the final 60 minutes of the bright-light exposure was 33 percent or more below that during the corresponding 60-minute interval 24 hours earlier. On two occasions Patients 1 and 2 were exposed to bright light for three hours; on one of those occasions opaque patches were used to shield their eyes from the light.

Evaluation of Entrainment by Repeated Phase Assessments

The blind patients were asked to return for repeated phase assessments of body temperature and melatonin secretion during a constant routine to evaluate the entrainment of their circadian pacemakers to the 24-hour day. Since Patient 1 was unwilling to undergo a second constant routine, blood samples for the plasma melatonin assay were collected in dim light (approximately 10 to 15 lux) with the patient kept in a semirecumbent position, to evaluate entrainment.7,19

Lighting Conditions

The intensity of ambient light, provided by ceiling-mounted “cool white,” high-output fluorescent lamps (North American Philips Lighting, Bloomfield, N.J.), was approximately 150 lux (equivalent to ordinary indoor artificial light) during the base-line day, approximately 10 to 15 lux (equivalent to dim indoor light) during the constant routines, 6000 to 13,700 lux (equivalent to ambient outdoor light just after dawn) during the exposure to bright light in the melatonin-suppression tests (both the 90-to-100-minute and the 3-hour tests), and 0.02 lux or less (equivalent to total darkness) during sleep. Light levels during the melatonin-suppression tests were recorded every 5 to 10 minutes by photometers (International Light, Newburyport, Mass., and Sper Scientific, Tempe, Ariz.) placed on the forehead and directed toward the angle of gaze.

Physiologic Measures

Core body temperature was continuously recorded by a rectal thermistor (Yellow Springs Instrument, Yellow Springs, Ohio). Blood samples were collected every 10 to 60 minutes through an intravenous catheter in the subject's forearm. Plasma melatonin was measured with radioimmunoassay kits (Elias USA, Osceola, Wis.; assay sensitivity, 7 pmol per liter; intraassay and interassay coefficients of variation, 8 and 15 percent, respectively) or (in the case of Patients 4 and 5) by the method of Arendt et al.25 All the samples collected from a subject during a single assessment were analyzed in the same assay.

Statistical Analysis

The fitted minimum of the body temperature and the fitted maximum of the plasma melatonin rhythms were used as markers of the phase of the endogenous circadian pacemaker.3,24 Nonlinear least-squares harmonic-regression analysis26 was used to fit a dual-harmonic model with correlated noise (for body temperature) or a one-harmonic model (for melatonin secretion) to the data. The results from the first five hours of the constant routine were excluded to eliminate masking effects produced by the preceding episode of sleep.