Sleep occurs in waves, with a trough called deep sleep when we are in maximum recovery mode. Its nature is somewhat less well known than the more dramatic "dream sleep” during REM. During deep sleep, blood pressure drops, breathing may become slightly slower, and muscles are relaxed as tissue growth and repair occurs and energy is restored. Most deep sleep occurs during the first two sleep cycles of the night, with the greatest amount typically occurring in the first cycle. As the night progresses, deep sleep decreases and is replaced by an increasing amount of REM sleep toward morning.
Deep sleep is effective in decreasing the sleep drive that builds steadily over the course of the day. Short afternoon naps of about 20-25 minutes or less do not allow enough time to cycle into deep sleep and therefore are less likely to result in the difficulty falling asleep that longer naps can produce.
Our access to deep sleep is vulnerable to the effects of stress, sleep disruption, lack of physical exercise, timing of meals, and caffeine consumption, among other things. Without adequate deep sleep, these factors contribute to the run-down feelings people may frequently experience.
A few suggestions for increasing deep sleep: