xl. The post-lunch dip

6 June 2008 at 16:10 | Posted in Circadian rhythm | 5 Comments

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”.


Next:  xli. Coincidence & update



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  1. OMG, this describes me perfectly. I was wondering why I keep wanting to nap at 7 or 8 pm, even if I went to bed at 4am and got up at 11am or noon. IT MAKES SENSE NOW! \o/

  2. I also have this dip at 8pm regularly and now this is even starting to make more sense to me!

    Body clock = cortisol rhythm

    And it also just hit me why people with DSPS have a longer interval between nadir and wake up time, this is caused due to the lack of cortisol production, in normal people cortisol production starts around 5am and reaches its peak around 7am which results in spontaneous waking up when the sun comes up & feeling extremely hungry.

    For people not familiar with cortisol, cortisol is called the anti-stress hormone, and our body should produce it during the daytime to keep us alert & to be able to instantaneously respond to dangerous situations that we might encounter during our day, that’s why they also call it the fight of flight hormone.

  3. Btw, this might also be the reason why all treatments of DSPS only have a moderate success rate, or why the success disappears rather quickly after some time, because only the day/night cycle (melatonin production) is adjusted by these treatments and not the body clock (cortisol production)!

  4. There are body clocks in many organs. They are all supposed to be coordinated by the main one in the suprachiasmatic nuclei. Severe jet lag is the lack of this coordination for a time; I’ve read more than once that the liver takes the longest time to catch up. It seems reasonable to me that people with circadian rhythm disorders may suffer this lack of coordination, at least at times.

  5. So why is DSPS treated by trying to adjust the melatonin production only and not the body clocks of all our other organs? I wouldn’t be surprised if one of those other clocks influences the melatonin production as well, and trying to shift the time of melatonin production might only help temporary until one of those other clocks (re)sets it back to the original values from before the manual adjustment/manipulation? And I wouldn’t be surprised if the clock that is responsible for our cortisol production (also light dependent, but in a slightly different way than melatonine as the control mechanism only listens to daylight while cortisol production also listens to indoor/artificial light) is to blame…

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