6.6.2.1. JET-LAG

Changes in sleep-wake cycle times related to phase modifications in environmental synchronizers.
 
6.6.2.1.1. Westward flights 

– Bed time is delayed so that sleep period coincides with local physical night.

– The first night after arrival depicts several modifications with regard to normal sleep. There is more REM sleep in the first part than normal and this tends to be interrupted by awakening episodes in the second half. The result is diurnal sleepiness.

– Normal sleep is generally recovered itself after three nights.

– When layover is short (< 48 hours), it is preferable to keep normal bedtimes and rising times in order to avoid disturbances related to the first night.

– When the layover is long (> 48 hours), it is preferable to synchronize with local time by going to bed as late as possible as from the first night. 
 

6.6.2.1.2. Eastward flights 

– The first night's sleep is sometimes better than normal, provided that the subject did not sleep during the flight and did not take a nap upon arrival. He or she will have accumulated a sufficient sleep debt to compensate for the jet-lag effects.

– These effects show themselves during the following nights by more unstable sleep with more interruptions.

– An improvement in the quality of sleep will occur after a period of around seven days.

– In Eastward rotations a short layover (< 48 hours) is therefore preferable in order to stay synchronized with home time. 
 
 
 
 
 

6.6.2.2. NIGHT WORK & ALTERNATING WORK SCHEDULES

Changes in sleep-wake cycle times are related to work schedules.
 
Coping with disturbances in sleep related to shift work is often more difficult than with those related to jet-lag. In fact, the environmental synchronizers, as stated earlier, tend to keep the circadian system on a par with standard time.
 
The result is a perpetual conflict between biological clock phase and the sleep period imposed by working hours.
 
Both quality and quantity of sleep taken during the day after a night shift are insufficient. Sleep occurs during a period of the day that is unfavorable to sleep both from a biological (ascending body temperature phase) and from an environmental point of view (light, noise, and social rhythms).
 
Night sleep before a morning shift is shorter due to early wake-up. The zones of least drowsiness located at the start of the evening, as well as social pressures do not lend themselves for sleep time to be advanced. Going to bed earlier serves no purpose, as the onset of sleep will not be much earlier than usual.
6.6.3. NATURAL PREVENTION OF DISORDERS RELATED  TO CHANGES IN SLEEP-WAKE TIMES 
　
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