All people have the deepest part of their sleep during the first third of the night. And during this period we have a faster transition into deep sleep. This is the sleep phase where we are less likely to wake up; our ‘arousal threshold’ is at highest level. Non-REM Stage 3 and Stage 4 together form a deep sleep, which is a form of synchronized sleep as the brain's electrical activity settles to a harmonized rhythm.
The fact is, deep sleep is related to the amount of prior time spent awake. For example, if you have a nap during the late afternoon for a long time, the time spent in slow-wave sleep (SWS) during the subsequent night is reduced. And if you are sleep-deprived for 36 hours, then during subsequent sleep the amount of time spent in SWS is increased (as reward for the accumulated sleep debt).
In this case, sleep is ‘homeostatic’. It is also possible to measure the intensity of deep sleep through looking at the power of slow-waves, measured as slow-wave activity. It was found that both slow-wave sleep minutes and slow-wave activity noticeably decrease with age. On average, women have more SWS than men, although there are large individual differences in the duration of SWS.
Slow-wave sleep was most often associated with memory consolidation, ability and vigilance the next day. A new study shows that SWS suppression in healthy adults leads to reduced insulin sensitivity, highlighting a potential link between SWS disturbance and increased risk of diabetes. Moreover, sleep and psychiatric disorders, including insomnia were associated with reductions in SWS.
It was found that night-time stress, pre-sleep were associated with reduced SWS once asleep. The expression of SWS is associated with low levels of the stress hormone cortisol, as well as reductions in sympathetic nervous system activity and increased parasympathetic nervous system activity. Experimental research confirms that SWS is involved in critical aspects of cognition and daytime functioning, and that it helps in optimally maintaining many functions of the brain and body.