Tags: Body clock, Circadian rhythm, Hypnogram, Nap, Sleep architecture, Sleep research, Sleep stages
Sleep researcher Sara C. Mednick has written the book Take a Nap! Change your life. (Workman Publishing, NY, 2006)
Yes, it continues in the over-enthusiastic, cheer-leading tone seen in the title, and no, it doesn’t address circadian rhythm disorders. The book is so popularized that it doesn’t even have an index. But it’s interesting and it’s based on solid science. It tells about sleep stages, what they are for and their circadian rhythms.
Though we spend about half our total sleep time in Stage 2 sleep, it’s been known for some time that most deep sleep appears in the first half of one’s night, and most rapid eye movement (REM) sleep appears in the last half. Here’s a hypnogram from Wikipedia showing a normal night’s sleep with its sleep stages:
Deep sleep (slow wave sleep) is stages 3 and 4 where Stage 3 includes 20-50% delta waves and Stage 4 includes over 50% delta waves. The illustration clearly shows deep sleep in the early part of the night (more clearly than a more modern one might, as stages 3 and 4 often are combined into one, at least in the USA).
It would never have occurred to me, but Dr. Mednick has shown that this early night/late night division into predominantly deep sleep and predominantly REM sleep is just part of a whole circadian cycle: a morning nap will include more REM sleep while an afternoon/evening nap offers more slow wave sleep. Sleep cycles generally contain the lowest amount of REM a couple of hours before bedtime and the greatest amount twelve hours later. I’ve tried to illustrate this here:
And should we care which sleep stages we are getting in a nap? In a nap of 20 minutes or less, the answer is no, as that nap includes only stages 1 and 2. But in a longer nap, up to an hour and a half or so, we go through a whole sleep cycle and it may indeed matter which type of sleep we are wanting.
The short “Stage 2 nap” increases alertness, stamina and physical dexterity. Drowsy drivers have a lot to gain from a 15-20-minute nap.
The morning nap, with more REM-sleep, inspires creative insight, heightens sensory perception and consolidates newly learned material including spacial orientation. The evening nap, with more deep sleep, provides tissue repair, improves memory and clears the mind (“prunes deadwood”).
On one point, Dr. Mednick disagrees with other experts. She says a nap no longer than three hours ending no later than three hours before bedtime will not interfere with nocturnal sleep and may even improve it!
Take a Nap! provides detailed instructions for planning your ideal nap. NB! if you are not well-rested, sleep debt will demand the repaying of SWS first, whether or not that’s what you want most of. See also Mednick’s website: www.takeanap.info
( posted by D )
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