. a nap of one hour can include a REM sleep period (small amount),

. if length of naps is two hours or more, REM sleep occurs, at the    earliest 50 minutes after nap onset and its duration is higher than    that of the deep slow-wave sleep.

After a prolonged period of activity (e.g.: 30 hours) deep slow-wave sleep is predominant irrespective of the time of day in which the nap is taken.
 
6.5.2.4. EFFECTS OF NAPPING

6.5.2.4.1. Nocturnal sleep following a nap

– Diurnal nap:

. The time slot in which the nap takes place determines the repercussions that will occur during the following night's sleep;

. A nap at the start of the afternoon delays sleep onset and reduces the duration of the deep slow-wave sleep during the subsequent night;

. The REM sleep predominant in the morning nap (8am to 10am) will neither influence the composition nor the duration of the subsequent night's sleep.

– Nocturnal nap:

. A nocturnal nap implicitly supposes sleep loss,

. The wakefulness period between the nocturnal nap and next night's sleep is increased,

. Sleep latency is low on the subsequent night,

. Sleep architecture is maintained (90-minute cycle),

. Increase (rebound effect) in the duration of the various sleep stages:

– The first nocturnal sleep will be very rich in deep slow-wave sleep    without change or even with a reduction in REM sleep,

– The REM sleep rebound occurs during the second night or even during subsequent ones.

It seems, therefore, that the need for deep slow-wave sleep takes precedence over the need for other types of sleep.
 
 
 
 
6.5.2.4.2. Sleep inertia: immediate effect of taking a nap 

6.5.2.4.2.1 Definition 
Sleep inertia leads to a transient state of disorientation or mental confusion upon waking up. It occurs irrespective of the time of day and even in subjects not suffering from sleep loss.
 
Temporary symptoms are degradation in mental performance and an alteration in mood.
 
While its length normally varies from 5 to 15 minutes it can reach up to several hours in case of high sleep debts.
 
 

6.5.2.4.2.2 Factors increasing sleep inertia 

– Sleep stage in which wake-up occurs: sleep inertia is very high when awakening suddenly occurs during a deep slow-wave sleep phase. 

– Length of deep slow-wave sleeps: the longer the deep slow-wave sleeps period, the higher the inertia. 

– Length of previous wakefulness period: high sleep loss leads to an increase in the duration of the deep slow-wave sleep and, as a result, sleep inertia increases from 25 minutes to several hours. 

– Nap time: in case of sleep loss (45 hours), an early morning nap (4am – 6am) leads to very high inertia which can last for several hours. However, inertia is attenuated to a great extent when the nap is taken in the afternoon (12pm – 2pm), even if sleep loss is higher (53 hours). 
　
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