Sleep Stages and Cycles

Taking 4 two hour naps isn't the same as taking an eight hour sleep. Why? Because of the body's sleep cycle as it bounces between Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM) sleep.

NREM is essentially dreamless sleep. During which the blood pressure is low, sleep breathing and heart rate are slow and regular. There are different stages of NREM sleep depicting the different depths. (medicinenet.com)

REM is when dreams usually occur. During which the heartbeat and blood pressure is more irregular. It is also usually accompanied  by involuntary muscle jerks.  (medicinenet.com)

When you first fall asleep you first enter NREM1 (~1-7mins), then NREM2 (10-25mins), and NREM3 (20-40mins) where sleep inertia is the strongest. After reaching deep sleep you will ascend back to NREM2 (~5-10mins) then finally into REM sleep. Your sleep will cycle back and forth like this.

Seen below is a pie chart on the average total time spent in each stage of the sleep cycle. However it isn't as simple as dividing the time spent in each stage between the cycles as each cycle will have different proportions of REM and NREM sleep depending on the state of the person.

http://www.howsleepworks.com/types_cycles.html

As you can see, NREM2 takes up a surprisingly large amount of the total time spent sleeping despite the relative benefit it produces. This is because it is the transition phase between deep sleep and REM.

Different stages of the cycle serve different restorative purposes, so the amount of time spent in each stage during each consecutive cycle differs as the night progresses.

Stages	When	What	Benefits
REM	After the ascension from NREM3	* Brain is active and dreams occur * Eyes dart back and forth * Body becomes immobile and relaxed, as muscles are turned off	* Provides energy to brain and body * Supports daytime performance
NREM 1	Occurs first	Light sleep	Nothing
NREM 2	Between phase of NREM 1 and 3	* Becoming disengaged from surroundings * Breathing and heart rate are regular * Body temperature drops (so sleeping in a cool room is helpful	Nothing
NREM 3 & 4	The bottom of the sleep phase	* Deepest and Most restorative stage of sleep * Blood pressure drops * Breathing becomes slower * Muscles are relaxed * Blood supply to muscles increases	* Tissue growth and repair occurs * Energy is restored * Hormones are released, such as: Growth hormone, essential for growth and development, including muscle development

http://sleepfoundation.org/how-sleep-works/what-happens-when-you-sleep

As you can see in the graph below, the amount of time spent in NREM3 stage decreases and time spent in REM increases as the night progresses. Probably explaining why it is much easier to wake up after a long sleep than a short one as you are statistically more likely to catch yourself in the light sleep stages.

http://www.quantadynamics.com/research/performancerem.htm

So to maximize the benefits of time spent sleeping it is better to sleep for an extended period of time than to take multiple naps throughout the day. That being said, beware of oversleeping!