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Stage 1 Sleep: This is experienced as falling to sleep and is a transition stage between wake and sleep. It usually lasts between 1 and 5 minutes and occupies approximately 2-5 % of a normal night of sleep. This stage is dramatically increased in some insomnia (restless legs) and disorders that produce frequent arousals such as apnea and PLMS.
Stage 2 Sleep: This follows Stage 1 sleep and is the "baseline" of sleep. This stage is part of the 90 minute cycle and occupies approximately 45-60% of sleep.
Stage 3 and 4 or Delta sleep: Stage 2 sleep evolves into "Delta" sleep or "slow wave" sleep in approximately 10-20 minutes and may last 15-30 minutes. It is called "slow wave" sleep because brain activity slows down dramatically from the "theta" rhythm of Stage 2 to a much slower rhythm of 1 to 2 cycles per second called "delta" and the height or amplitude of the waves increases dramatically. In most adults these two stages are 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 restorative. It is delta sleep that a sleep-deprived person's brain craves the first and foremost. In children, delta sleep can occupy up to 40% of all sleep time and this is what makes children unwakeable or "dead asleep" during most of the night.
Stage 5: REM (Rapid Eye Movement Sleep): This is a very active stage of sleep. Composes 20-25 % of a normal nights sleep. Breathing, heart rate and brain wave activity quicken. Vivid Dreams can occur. Sleep Specialists call this 5th stage of sleep "REM" rapid eye movement sleep because if one is to watch a person in this stage, their eyes are moving rapidly about. After REM stage, the body usually returns to Stage 2 sleep.