These differences are less obvious during working days but become more marked during holidays and days-off: evening types delay their bedtimes considerably and extend rising times when so
6.5.1.3. FUNCTIONS 

REM sleep is said to be involved in the restoration of attention, learning, memory, emotional balance and mood mechanisms.
 
Deep slow-wave sleep apparently allows reconstruction or maintenance of physical integrity: nocturnal increase in protein synthesis and cell division.
 
6.5.1.4. ORIGIN 

It is sometimes implicitly assumed that nocturnal sleep onset is simply related to habit. However, when there are no "direct time givers (watch, radio, newspapers, etc.) or "indirect time givers" (lighting, noise, temperature, vibrations, etc.), an isolated subject maintains his wake-sleep cycle rhythmicity but with a period of around 25 hours instead of 24 hours. This shift leads to increasingly later rising times and bedtimes.
 
The persistence of the sleep-wake periodicity reflects the existence of an internal system, or biological clock, which functions autonomously and has its own frequency.
 
Under normal conditions, the circadian clock is driven or synchronized to a period equal to 24 hours by periodic variations called environmental synchronizers.
 
The 25-hour cycle suggests that when they are no constraints, human beings have a natural tendency to lag behind the environmental synchronizers.
 
Because of this spontaneous tendency, it is easier for a traveler to delay both his bedtimes and rising times after a westward flight than to advance them for an eastward flight.
 
6.5.1.5. RELATIONS BETWEEN SLEEP & BIOLOGICAL CLOCK PHASE

Sleep latency, duration and composition depend on body temperature cycle where sleep onset is located.
 

 
6.5.1.5.1. If sleep starts a little after maximum temperature, i.e. around 11pm 

. sleep duration is long and awakening occurs during the ascending part of the thermal cycle,

. sleep architecture is normal:

– REM stage occurs every 90 to 120 minutes,

– REM sleep episode duration is higher in the second half of the night,

– deep slow-wave sleep (Stages 3 and 4) is concentrated within the first half of the night. 
 
 
 
 
 
 
 
 

6.5.1.5.2. If sleep starts around the thermal minimum, i.e. around 5am

. drowsiness is extreme with low sleep latency,

. REM sleep is facilitated (reduction in latency of first episode and increase in its duration),

. sleep will be relatively short as wake-up occurs as soon as body temperature starts rising. 

 
　
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