Q. I typically read at night and fall asleep, if I do, about 2 o’clock in the morning and wake again after just a few hours of sleep. Someone told me the other day that I should be getting to sleep by 10 o’clock at night. Is there any evidence for this recommendation whatsoever?

A. In general, the human brain is believed to follow naturally a circadian cycle of about 10:00 pm to 5:00 or 6:00 am for sleep. Typically, the human sleep pattern is thought to occur in 90-minute cycles of non-REM (rapid eye motion) and REM sleep. These cycles are not completed balanced in composition however. The first 2-3 cycles contain more deep non-REM sleep, while the remaining 2-3 cycles are comprised of lighter REM sleep.

According to Matthew P. Walker, PhD, director of the Sleep and Neuroimaging Lab at Beth Israel Deaconess Medical Center and Assistant Professor of Psychology at Harvard Medical School, the ratio of non-REM-to-REM sleep within the brain’s 90-minute cycles changes across the night. He explains that early in the night (11p.m.-3a.m.), the majority of those cycles are comprised of deep non-REM sleep (stages 3 and 4) and very little REM sleep.

During the second half of the night (3 a.m.-7 a.m.), the 90-minute cycles tend toward more REM sleep (the stage commonly associated with dreaming), together with a lighter form of non-REM sleep (stage 2). He also explains that there is a subtle twist on this: the earlier in the night, the greater the propensity for deep non-REM sleep, and the later in the morning, the greater the propensity for REM sleep. This means that a person who sleeps 8 hours from 9p.m. to 5a.m will have a different overall composition of sleep—biased towards more non-REM—than someone who sleeps from 3a.m. to 11a.m., who is likely to experience more REM sleep ( although the total amount of sleep may still equal 8 hours).

Although I used to keep proverbial night-owl hours, especially on weekend days, often sleeping from midnight to eight or nine in the morning. That is no longer the case. My pattern now is to be falling asleep by 10:00 p.m. and waking up in the morning naturally (without an alarm clock) between six and seven. I feel more rested and find my brain much more inclined to “write” during the morning hours. Who knew?