Sleep is not uniform. During sleep our body cycles through four different stages (each with specific brain activities) consisting of both Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM) sleep. On average each cycle lasts about 90 minutes. On a normal night you may go through four to six cycles. The first cycle is usually the shortest. As the night progresses, fewer NREM stages occur, and the duration of REM sleep episodes increases (3) .  
 
There are two main types of sleep: 

• Rapid Eye Movement (REM) sleep 
• Non-Rapid Eye Movement (NREM) sleep, which has three distinct stages. The higher the stage of NREM sleep, the harder it is to wake up a person from. 

People normally cycle through the three stages of NREM sleep (stages N1 through N3), usually followed by a brief interval of REM sleep, every 90 to 120 minutes or several times every night. Throughout the night, people wake up briefly (called stage W) but are typically unaware of being awake. 

As mentioned, the composition of each cycle — how much time is spent in each sleep stage — changes as the night goes along.  Sleep cycles also vary from person to person and from night to night (4) based on a wide range of factors such as age, recent sleep patterns, physical activity, and drug or alcohol consumption. Certain medications can impact sleep composition. 

NREM sleep:  Sleep progresses from Stage 1 (the lightest level, when the sleeper can be awakened easily) to stage 3 (the deepest level, when greater effort is necessary for the sleeper to be awakened ). In stage 3, blood pressure is at its lowest, and heart and breathing rates are at their slowest. Sleep experts perceive Stage 3 as high-quality sleep. The older we get, however, the amount of Stage 3 decreases. A simple measuring of this is noticing how much more energy a child has as compared to an adult or senior citizen. 

REM sleep: Electrical activity in the brain is unusually high, somewhat resembling that during wakefulness. The eyes move rapidly, and certain muscles are paralyzed so that voluntary movement is impossible. However, some muscles may twitch involuntarily. The rate and depth of breathing increase. 

 
The four Stages of Sleep are: 

• Stage N1 (NREM 1): Transition period from being awake to falling asleep.  During this time, you may have a sudden dream onset.  You are drifting off to sleep and may still be aware of your surroundings and easily be aroused back to wakefulness. There is muscle tone present in the skeletal muscle, and breathing tends to occur at a regular rate. This stage tends to last 1 to 5 minutes, consisting of around 5% of the total cycle. 
• Stage N2 (NREM 2): This stage represents deeper sleep as your heart rate and body temperate drop, and muscles relax.  Stage N2 sleep lasts around 25 minutes in the initial cycle and lengthens with each successive cycle, eventually consisting of about 50% of total sleep. 
• Stage N3 (NREM 3): This is considered the deepest stage of sleep. This stage refreshes the body (vs brain and memory refresh in Stage REM). In N3, muscle tone, pulse, and breathing rates decrease as the body relaxes even further. This is the regenerative stage of sleep, when the body repairs and regrows its tissues, builds bone and muscle, and strengthens the immune system.   As people get older, they tend to spend less time in this deep sleep and more time in stage N2 sleep. This is normal and by itself doesn’t indicate a problem. 
• Stage R (REM): “rapid eye movement” sleep. During this stage, the brain, memory, and cognitive abilities are refreshed. Without REM sleep, an individual may feel brain fog the next day.  Heart and breathing rates increase and become irregular.  It is during REM sleep that we dream.   
• Many of us will remember dreams from the REM stage.  The body creates chemicals that render us temporarily paralyzed so that we do not act out our dreams. In this stage, the brain is extremely active, and our eyes, although closed, dart back and forth as if we were awake.  
• People enter REM sleep within the first 90 minutes of falling asleep and, as the sleep cycle repeats throughout the night, REM sleep occurs several times nightly. It accounts for approximately 20 to 25 percent of an adult’s sleep cycle, and over 50 percent of an infant’s. The first phase of REM usually lasts for 10 minutes, with each phase getting progressively longer (5). The final phase of REM sleep may last for up to an hour. Most dreams occur during REM sleep, and it plays a fundamental role in our cognitive activities: learning, consolidation of memory, and mood. 

The breakdown of a person’s sleep into various cycles and stages is commonly referred to as their sleep architecture. During a diagnostic overnight sleep study, or polysomnogram, your sleep architecture and some other bodily functions are tracked and recorded.  People with apnea may have reduced Stages N3 and REM when interruptions in breathing cause sleep to be fragmented, possibly alternating between stages N1 and N2 over and over all night. This deeply affects the quality of sleep.  

References 

1. Jehan S, Zizi F, Pandi-Perumal SR, et al. Obstructive Sleep Apnea and Obesity: Implications for Public Health. Sleep Med Disord. 2017;1(4):00019. 

1. Paruthi S, Brooks LJ, D’Ambrosio C, Hall WA, Kotagal S, Lloyd RM, et al. Recommended amount of sleep for pediatric populations: a consensus statement of the American Academy of Sleep Medicine. J Clin Sleep Med. 2016;12(6):785–786. 

1. Watson NF, Badr MS, Belenky G, et al. Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep. 2015;38(6):843–844. 

1. Watson, Nathaniel F. “Health Care Savings: The Economic Value of Diagnostic and Therapeutic Care for Obstructive Sleep Apnea.” Journal of clinical sleep medicine: JCSM : official publication of the American Academy of Sleep Medicine vol. 12,8 1075-7. 15 Aug. 2016, doi:10.5664/jcsm.6034 

1. National Institute of Neurological Disorders and Stroke. (n.d.). Brain basics: Understanding sleep.