Strogatz, Steven. Sync, 1st edition (New York, New York: Hyperion, 2003)
Chapter 3, "Sleep and the Daily Struggle for Sync"
- two internal cycles:
- body temperature
- sleep cycle
- under normal conditions, sleep and temperature cycles locked together
- human body has a "intrinsic" 26-hour cycle
- 26-hour cycle manifests itself in the absence of external cues (daylight)
- external cues force period into 24-hour cycle
- temp cycle
- sinusoid: period=24-26 hrs, amplitude = 98.6 +/- 1.5 deg F
- sleep cycle
- locked with temp cycle, unless desync occurs from lack of external cues (sunlight)
- go to sleep when temperature is at minimum
...For people who are entrained to the 24-hour day, body temperature typically reaches its lowest point about 1 or 2 hours before the time of habitual wake-up. For example, much of the labor force wakes up at around 6 or 7 A.M. Hence, for those people, minimum body temperature probably occurs between 4 and 6 A.M. the jump in sleep duration is predicted to occur about 9-10 hours after that, which translates to a clock time of 1-4 P.M. As claimed, that's nap time.
- REM sleep
- also synchronized with body temperature
- usually occur in early morning
- forbidden sleep zones
- graph relative frquencies of sleep onsets as a function of circadian phase
- two valleys: 2-3 hours wide, centered ~5 hours after and ~8 hrs before time of min temperature
So for someone who sleeps form 11 P.M. to 7 A.M. each night, the data predicted a "morning forbidden zone" at around 10-11 A.M., and an "evening forbidden zone" at around 9-10 P.M., just an hour or two before bedtime.
The distribution also showed two peaks, representing the sleepiest times in the cycle, in the sense that these were the bedtimes the subjects selected most often (without realizing it, of course, since they were in time isolation). A broad peak centered around the temperature trough coincided with the zombie zone, indicating that this window of minimum alertness was also the time of maximum sleepiness. A second peak occurred about 9-10 hours after minimum temperature, corresponding to siesta time, 2-3 P.M. in the outside world.
- body can adjust circadian cycle only so far
- an individual may not be able to force their period to 24 hours
- results: sleep disorder
- forbidden zone periodically coincides with bedtime
- hypothalamus sets period and phase
- linked to photoreceptors in the eyes
- melatonin
- tied to circadian rhythm
- produced by pineal gland
- maximum output during sleep
- output plummets when light enter eyes
