Fig. 1. The transition from wakefulness to stage 1 sleep. The obvious signal of the nearly instantaneous transition is the abrupt cessation of rhythmic alpha activity (8–12 cps) followed by a low-amplitude, mixed-frequency pattern. The change in EEG patterns to stage 1 as illustrated here is designated the onset of sleep. Therefore, the moment of sleep occurs at the arrow. The second arrow represents the moment the test should be terminated.

Fig. 2. The results of an idealized MSLT. The speed of falling asleep is related to the amount of sleep on prior nights. A standard series of calibrations are performed while the subject is awake that require about a minute. Then the subject is given the instruction, “close your eyes and try to go to sleep.” The precise second the lights are out, the sleep latency measure begins. The five individual test results connected by a line comprise a “sleep latency profile.” Examining the profile can give an overall sense of daytime sleep tendency. Alternatively, the mean of the five individual tests can also be used as a descriptor of the overall strength of the daytime sleep tendency on this particular day. It must always be kept in mind that the individual test is terminated within seconds after the onset of sleep. It is extremely rare that it takes more than 30 seconds for an observer to make this decision. Each test is always terminated if the subject does not fall asleep in 20 minutes.

Fig. 3. There is a general correspondence between the MSLT mean score and the overall level or degree of daytime alertness. If an individual falls asleep in less than 5 minutes on every test (sometimes less than a minute), this indicates a very strong sleep tendency and a very strong sleep drive. The label “twilight zone” is meaningful in this respect because memory and clarity of thinking is usually substantially impaired in individuals whose MSLT score is less than 5.

Fig. 4. MSLT profiles after varying amounts of nocturnal sleep. This figure is based on a very large amount of data. The MSLT profiles and mean scores are directly related to the amount of sleep at night. In these tests, the scheduled time in bed varied from 10 hours to zero. Generally, with 10 hours in bed, total sleep time was 9 hours or more. With the lower values, total sleep time closely approached total time in bed.

Fig. 5. Cumulative effects of sleep deprivation. This figure summarizes the first experiment demonstrating that sleep loss accumulates. Ten young adults participated. After three baseline measurements with 9 hours in bed for three consecutive nights, the MSLT was performed on each successive day. Because time out of bed increased each day by 4 hours, an additional sleep latency test (total 6) was added. Each point in the graph is the grand mean of ten daily mean MSLT scores, or a grand total of 50 measurements each day during baseline, and 60 measurements each day during restriction. As can be clearly seen there is a progressive decrease in the overall sleep latency, indicating a progressive increase in daytime sleep tendency. The vertical bars indicate the standard deviation. These results led directly to the very important concept of sleep debt. Other researchers have since replicated and confirmed the findings. Although there is often considerable individual variation, no one has reported a single instance of the failure of sleep tendency to increase with accumulating sleep debt.

Fig. 6. This graph illustrates the results in a single individual during the baseline MSLT. The sleep latency values are in the area of good alertness.

Fig. 7. This graph illustrates the results in a single individual on Friday after only five nights of sleep restriction. The MSLT profile on Friday is well within the twilight zone even though the baseline score is at the level of good alertness. Most of the subjects did not show a rapid decrease in sleep latency scores but all showed a significant decrease.

Fig. 8. Typical subject in the 14 hours in bed study. The last 7 nights of the experimental period are not shown because there was no further change in total sleep time. Total sleep time on the first 4 nights of the 14-hour schedule is 12 hours or more. There is an asymptotic decline in total sleep time which levels off at about night 21. For the next 2 weeks the nightly total sleep time hovered around the mean for the group (8 hours 15 minutes). The sleep debt figure is on the low side because the 8 hour and 15 minute amount is obtained in a very inefficient manner and is probably fragmented and light as a result of the 14 hours in bed. The major point is that when the sleep debt is completely repaid no extra sleep can be obtained, and even having to lie in the dark awake for 5 hours and 45 minutes does not enable extra sleep to occur. The 8 hours and 15 minutes could be considered this individual's daily sleep requirement, although it should be set somewhat lower in a real world situation.