What Happens During Your Sleep Cycle?

Sleep progresses in a series of four or five sleep cycles of non-REM and REM sleep throughout the night. The first sleep cycle is typically around 90 minutes in length, with the succeeding cycles averaging around 100-120 minutes, although they are usually shorter in children. Each cycle progresses from stage 1 to 2 then to three (3 and 4) before reversing back up to stage three then stage two and stage one before entering  a short period of REM sleep, before going back through the stages (stage 1 – stage 2 – stage 3 and 4) in a new cycle. As the night progresses, the time spent in deep stage 3 and stage 4 sleep decreases and the time spent in REM sleep increases, so that there is a greater proportion of stage 3 and stage 4 sleep earlier in the night, and a greater proportion of REM sleep later in the night, particularly during the final two sleep cycles.

 Each sleep stage in any particular sleep cycle fulfills a distinct physiological and neurological function, each of which appears to be necessary for the health of the body and mind, to the extent that, if sleep is interrupted or if certain stages are missing for any reason, their physiological functions are not fully executed, and the person may feel tired or groggy even after an apparently sufficient sleep period, a phenomenon known as “sleep inertia”.  Other effects of insufficient sleep include: Weight gain, reduced overall functioning, Fatigue, Irritability, sleepiness throughout the day,  increase in errors, Less creativity, Immune system problems, Skin problems.  Defined below are the three main stages of sleep.

The Stages of Sleep

STAGE ONE (NREM1 or N1) : Between  wakefulness and sleep.  The muscles are still quite active and the eyes roll around slowly and may open and close from time to time. It is the period of transition from relatively unsynchronized beta and gamma brain waves to more synchronized but slower alpha waves and then to theta waves. During stage 1 sleep, breathing gradually becomes more regular and the heart rate begins to slow.  Hypnic  jerks are common. This stage usually lasts  less than 10 minutes, and you  may be aware of sounds and conversations. Typically, this stage represents only about 5% of the total sleep time.  Typically, this stage represents only about 5% of the total sleep time.

STAGE TWO (NREM2 or N2) : Muscle activity decreases still further and conscious awareness of the outside world begins to fade completely.  If any sounds are heard, the sleeper is not able to understand their content at this point.  Sleepers pass though this stage several times during the night – more time is spent in stage 2 sleep than in any other stage, (about 45%-50% of total sleep time for adults, or even more in young adults).

STAGES 3 and 4 (NREM3/or NREM4) is also known as deep or delta or slow-wave sleep (SWS): During this period the sleeper is even less responsive to the outside environment –  unaware of any sounds or other stimuli. This stage occurs in longer periods during the first half of the night, (during the first two sleep cycles).  Around 15%-20% of total adult sleep time.  Temperature, breathing rate, heart rate and blood pressure are all at their lowest levels during stage 3 sleep, and  dreaming is more common during this stage than in the other non-REM sleep stages, although not as common (nor as vivid and memorable) as during REM sleep.  Parasomnias (night terrors, sleep-walking, sleep-talking and bedwetting) occur here.  It is much more difficult to wake a person during stage 3 sleep, and if awakened at this stage they will often feel very groggy and may take up to 30 minutes before they attain normal mental performance (known as sleep inertia). Children and young adults tend to have more slow-wave stage 3 and 4 sleep than adults, and the elderly may experience little or no stage 3 and 4 sleep at all.

REM SLEEP: occurs in cycles of about 90-120 minutes throughout the night, and it accounts for up to 20-25% of total sleep time in adult humans, although the proportion decreases with age (a newborn baby may spend 80% of total sleep time in the REM stage). In particular, REM sleep dominates the latter half of the sleep period, especially the hours before waking, and the REM component of each sleep cycle typically increases as the night goes on.

The brain’s oxygen consumption, reflecting its energy consumption, is very high during this period, in fact often higher than when awake and working on a complex problem.  Breathing becomes more rapid and irregular during REM sleep than during non-REM sleep. In addition,  heart rate and blood pressure also increase to near waking levels.  Core temperature is not well regulated during this time and tends towards the ambient temperature, in much the same way as reptiles and other cold-blooded animals.

Although the muscles become more relaxed during non-REM sleep, they become completely paralyzed and unresponsive during REM sleep – ATONIA).  This occurs because the brain impulses that control muscle movement are completely suppressed (other than those controlling the eye movements and one or two other essential functions, like the heart, diaphragm, etc)

Why Rest Is So Important…

Lack of REM sleep has been shown to impair the ability to learn complex tasks, suggesting that REM sleep is a vital component of our sleep patterns, particularly during early childhood development, when REM sleep makes up a much larger percentage of total sleep.  Although most people do not tend to wake after each cycle of REM sleep, as some animals do, we are more likely to wake from REM sleep than from non-REM sleep. Usually, these “micro-awakenings” are of a few seconds only, and the sleeper does not normally remember them. If over-stimulated, though, a person may wake up fully, and it may take the length of an entire sleep cycle (1.5 – 2 hours) to get back to sleep.

The majority of dreams  occur during REM sleep. It is thought that the muscular atonia that accompanies it may be a built-in measure to protect us from self-damage which could occur while physically acting out these vivid REM dreams. If you have any additional inquiries, I will be more than happy to help.