Dijk and Czeisler 1995: REM Sleep Peaks at Your Habitual Wake Time
Eight men lived on a 28-hour day for 33-36 days. REM sleep was highest when a sleep episode ended at habitual wake time, and the study says little about alarms.
In this article4 sections
In a lab study with no clocks or daylight, REM sleep was highest when a sleep episode ended at the volunteer’s habitual wake time, compared with episodes that ended at other points on the body clock. That is the central result of Dijk and Czeisler (1995, The Journal of Neuroscience, 15: 3526-3538), and it is a statement about timing, not about how an alarm feels.
I could read the abstracts of this paper and its 1994 companion. The full text was behind a paywall, so every figure below comes from those abstracts.
What the experiment did
Eight men lived for 33 to 36 days in an environment without time cues. They followed a 28-hour rest-activity cycle, with each sleep episode lasting 9.33 hours. A 28-hour day is longer than the body clock can follow, so sleep landed at every phase of the clock’s own cycle, and the researchers could separate what the clock does from what time spent asleep does. The schedule also kept the amount of wakefulness before each sleep roughly constant (the authors’ words: variations in wakefulness preceding sleep were minimized). The 9.33-hour sleep is one third of 28 hours, which is my arithmetic.
If you want the history of this kind of study, five people who lived without clocks traces it from Mammoth Cave onward.
What they found
| Measure | Result in the abstract |
|---|---|
| REM sleep, circadian part | Crest shortly after the minimum of core body temperature |
| REM sleep, sleep-dependent part | Rose across the sleep episode |
| REM sleep, combined | Highest when the end of a sleep episode coincided with habitual wake time |
| Slow-wave activity in non-REM sleep | Fell across every episode; only a low-amplitude circadian rhythm that did not match sleep propensity |
| Sleep spindle activity | Rose across every episode; strong circadian rhythm, crest at the start of the habitual sleep episode |
Two clocks are involved, and they add up in an uneven way. The body clock pushes REM toward the hours just after the temperature minimum. Time spent asleep pushes REM up as the night goes on. At habitual wake time the two peaks overlap, so REM is at its highest there. The authors call the interaction nonadditive and conclude that the usual phase relation between the sleep-wake cycle and the clock “promotes the consolidation of sleep and wakefulness and facilitates the transitions between these vigilance states.”
The 1994 paper (Dijk and Czeisler, Neuroscience Letters, 166: 63-68) reached a related result from 175 polygraphically recorded sleep episodes. The body clock and the sleep homeostat, the pressure that builds with time awake, each contributed about equally to sleep consolidation.
Slow-wave activity, the EEG marker of deep sleep, behaved differently: it showed only a small circadian rhythm and did not follow the clock’s sleep-propensity curve.
What the paper does not say
Nobody was woken by an alarm. No one rated alertness on waking, so the abstract cannot tell you that REM is an easier or harder stage to wake from. The sleepers were eight men on an unusual schedule, so nothing here is a norm for women, for adolescents or for people on a 24-hour day. Linking the REM peak to sleep inertia would be an inference from other work, and the post on reading a sleep-stage chart is a better place to see how the stages are scored in the first place.
What to do tomorrow
These are inferences from the findings, not results of the paper.
- Keep wake time fixed. The REM peak appeared when sleep ended at the habitual position on the clock, and moving wake time moves that position. How quickly the position adapts after a change is what jet-lag recovery is about.
- Treat an alarm well before habitual wake time as cutting the night short before the REM peak, at a point where slow-wave activity has had less time to fall, since it declined across every episode. Whether that feels worse was not tested.
- Do not build a wake plan around counting 90-minute cycles. The abstract gives no cycle length and describes stage amounts changing with two clocks at once.
DontSnooze asks for 30 seconds of video proof when the alarm goes off, so it holds you to the clock time you chose. It cannot move you to the REM-rich moment this paper describes, and the paper does not say that moment would feel better.
The 1995 abstract’s last claim is the one to keep: the usual alignment between sleep and the clock is what the authors credit with consolidating sleep and wakefulness. A fixed wake time is the only part of that alignment most people control.