It is common for there to be a prolonged episode of wakefulness before take-off on space travel. There may be as long as 30h of continuous wakefulness before sleep is first obtained during the trip. It is usual for around 6h sleep in total to be taken each 24 h, but this is not consolidated into a single sleep episode.

The earth is encircled every 90min so that during this interval the external illumination is the equivalent of a normal day and night. Masks are therefore usually worn when attempting to sleep.

Sleep hygiene is difficult, particularly since there is little opportunity for physical exercise. Hypnotic use is common among astronauts. The weightlessness during space travel requires the sleeper to be attached to a point in the cabin to prevent drifting around. It also causes backache because of lengthening of the spinal column by 2–4cm due to reduced pressure on the intervertebral discs.

The environment in space reduces the gravitational effect on the structures in the upper airway. This reduces the upper airway resistance during sleep.

Snoring and sleep-disordered breathing are less likely and there are fewer respiratory arousals during sleep as a result. There is also a reduction in respiratory frequency and heart rate before sleep and in the latter during stages 3 and 4 NREM sleep.

Hyberbaric environments
Hyberbaric environments are usually encountered in workers on the sea-bed. Several people are usually present in a confined space and reduction of stages 3 and 4 NREM sleep, an increase in stages 1 and 2 and slight reduction of REM sleep are common. Sleep fragmentation is a problem and there is increased dream recall.

The extent of excessive daytime sleepiness is approximately proportional to the barometric pressure, especially at depths greater than 200–300m. The increased gas density leads to a sensation of nasal blockage which causes awakenings from sleep. Snoring and obstructive sleep apnoeas are more common than at sea level. There is also an initial polyuria which interrupts sleep and is due to haemodynamic changes.

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