We actually have the deepest part of our sleep during the first third of the night, and there is a more rapid transition into deep sleep during this period. This is the phase of sleep where we are the least likely to wake-up from, in other words, our 'arousal threshold' is at its highest. Non-REM Stage 3 and Stage 4 together make up deep sleep, sometimes called slow-wave sleep, because electroencephalography (EEG) reveals higher amplitude waves occurring at much lower frequencies. Deep sleep is a form of synchronized sleep because the brain's electrical activity settles to a harmonized rhythm.
Deep sleep is known to be related to the amount of prior time spent awake (sleep pressure). For example, if one naps during the late afternoon for a prolonged period, the time spent in slow-wave sleep (SWS) during the subsequent night is reduced. Similarly, if one is 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). Sleep is therefore 'homeostatic'. In addition to minutes of time spent in deep-sleep, it is also possible to measure the intensity of deep sleep through looking at the power of slow-waves, measured as slow-wave activity. Both slow-wave sleep minutes and slow-wave activity have been found to decrease markedly with age. On average, females have more SWS than men, though there are great individual differences in the duration of SWS.
Slow-wave sleep has most often been associated with memory consolidation, next-day learning ability and vigilance. Emerging work also suggests that SWS suppression in healthy adults leads to reduced insulin sensitivity, highlighting a potential link between SWS disturbance and increased risk of diabetes. Sleep and psychiatric disorders, including insomnia, have been associated with reductions in SWS. Night-time stress, pre-sleep, has also been found to be associated with reduced SWS once asleep. Indeed, 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. The emerging picture from experimental research is that SWS is involved in critical aspects of cognition and daytime functioning, and that it assists in the optimum maintenance of many brain and bodily functions.