xl. The post-lunch dip6 June 2008 at 16:10 | Posted in Circadian rhythm | 5 Comments
Tags: Body clock
Sleep timing, when not manipulated too badly, is controlled by body clock and homeostasis. Homeostasis starts building a need for sleep as soon as we wake up for the day, but at first it’s easy to ignore. For a normal or lark sleeper, that means until sometime between noon and two. At that point the body clock kicks with a need for wake which builds until just before bedtime. The sleep gate, a term used by some researchers, then opens and sleep occurs.
During about the first half of the night, one sleeps away the built-up sleep need (as controlled by homeostasis). Some people wake up for a time at that point, which researchers are starting to see is normal and not a problem.
Then, normally about 2 a.m., the body clock kicks in with an intense sleepiness which dissipates through the next 4 hours.
So a normal lark is sleepy at 10pm (body clock’s need for wake gets turned off). The day’s sleep debt is paid off by about 2am when the body clock turns on a need for four hours more sleep. Happy lark awakens at 6am, raring to go, when neither homeostasis nor body clock craves sleep.
In theory, the body clock should kick in with its need for wake just before a big sleepiness hits at 2pm, but there’s often a bit of a delay, probably culturally encouraged in areas using the siesta, and that included/includes many agrarian cultures far from Iberia. It is known that humans without electric light often experience an hour or two of wake in the middle of the night. (Said to be good for meditating, socializing and sex, not to mention nursing the baby.)
Thus, normal people are most intensely sleepy at 2pm and at 2am.
So! It seems reasonable to me to apply this 12 hour difference to people with Delayed Sleep-Phase Syndrome, at least those who aren’t totally desynchronized internally. Take the classical DSPS example of sleeping 4am to noon. At 8am, the body clock has just turned on its intense need for continued sleep (making it impossible to hear an alarm clock or three). At noon we awaken (not quite as bushy-tailed as the lark was at 8am, perhaps because society has told us all our lives that “sleeping in” is BAD!).
Twelve hours after the intense need for sleep at 8am, we may experience the sleepy “after-lunch dip” at around 8pm. All times are approximate.
This explains, I think, why the long naps I used to take at 6 or 7pm abruptly ended at 10pm or so. Very tired larks may be able to sleep a couple-three hours at siesta time, but they won’t sleep for 8 hours then. Same with me; if I haven’t been up the previous 36+ hours, I won’t sleep longer than about four hours at my “siesta time”.