Delta Sleep System
Delta sleep system is also referred as slow wave sleep.
It is the term used to describe the 3 and 4 stages of NREM sleep.
Delta sleep system occurs in these 3 and 4 stages of NREM.
In most adults these two stages are completed within the first two 90 minute sleep cycles or within the first three hours of sleep.
Let us look at the five stages of sleep in which delta sleep system occurs.
Five stages of sleep:
Stage 1: In stage 1, we enter the transition state as we fall asleep and begin our first "sleep cycle". This light sleep stage lasts up to 2-5 minutes. If sleep remains uninterrupted it will progress to stage 2 sleep. Stage 1 gives 2-5 % of normal sleep
Stage 2: Stage 2 is much deeper sleep than stage 1. The brain waves go into theta mode, and lead into stages 3 and 4 in around 10-20 minutes. Stage 2 sleep occupies approximately 50-65% of our sleep time, lasting 15-30 minutes in each cycle. During the second part of the night we spend more and more time alternating between stages 2 & REM sleep.
Stage 3: Stage 3 is much deeper sleep than stage 2. The muscles are relaxed, heart rate slows down, blood pressure falls, and breathing is steady and even. Brain activity slows down noticeably from the theta pattern of stage 2 to a much slower rhythm of 1 to 2 cycles per second called delta sleep system, and the height, or amplitude, of the waves increases.
Stage 4: Stage 4 is deepest sleep of all, during which a sleeping person is 'dead to the world'. Blood pressure and heart rate change and the sleeper's brain heats up. Delta sleep system is characterized by very high voltage slow brain waves.
Stage 5 REM (rapid eye movement): The first REM period lasts only about ten minutes. After that, the sleeper goes back into a deep stage 4 sleep or delta sleep system. Again, the sleeper returns into a REM stage after a short period, and cycles through REM and stage 4 continue until the sleeper wakes up.
Along with this, our brain becomes so activated that we start to fantasize and dream. Our eyes move as in alertness, and in relation to what we are dreaming. This paradoxical state will last 10-20 minutes and then we "fall" back down into stage 2 again.
This is the end of a sleep cycle and then it all starts over again. As the night progresses we slowly lose our delta sleep, and replace it with longer and longer periods of alternating stage 2 and REM sleep. By the final sleep cycle of the night, we spend approximately half our sleep time in stage 2 and half in REM.
NREM and REM states occur in a approximately 90 minute cycles, which is repeated 5 to 6 times a night. In most adults Stages 3 and 4, or Delta sleep, is completed within the first two 90 minute sleep cycles, or within the first three hours of sleep.
Contrary to popular belief, it is delta sleep that is the "deepest" stage of sleep (not REM) and the most soothing. It is delta sleep system that a sleep deprived person's brain desires the first and foremost. In children, delta sleep system can occupy up to 40% of all sleep time.