Do you know we spend one-third of our lives sleeping? A lot happens in our bodies when we sleep. Sleep is very important for our health and wellbeing. Getting enough quality sleep in the night is as important for survival as food and water.
Learning Objectives
- What is sleep?
- Sleep cycle and stages of sleep
- Differentiate between REM and Non-REM
- Areas of the brain and hormone secretions involved in sleep
- Various theories explaining the function of sleep
- Sleep debt and sleep deprivation
What is sleep?
Sleep is a biological necessity critical to maintaining mental and physical health. It is a state of lessened consciousness and decreases physical activity during which the organism slows down and repairs itself.
The sleep cycle is the alternate movement from light sleep to deep then deeper and deepest sleep throughout the sleep period. It involves the oscillation between the following two distinct phases:
- Rapid eye movement (REM) sleep during which dreaming occurs
- Non-rapid eye movement (NREM) sleep or slow-wave sleep
Sleep mechanisms
The sleep-wake cycle is regulated by an interplay of two mechanisms: sleep-homeostatic and circadian mechanisms.
- Circadian rhythms – It is the regulation of the body's internal processes and alertness levels governed by the internal biological or circadian clock. The circadian clock tells the body when to sleep and wake. It is controlled by a group of cells, called the Suprachiasmatic Nucleus (SCN) in the hypothalamus. SCN responds to light and dark signals. Light travels from the optic nerve of the eye and signals the internal clock that it is time to be awake. The SCN also signals to the other parts of the brain that control hormones, body temperature and other functions that play a role in making us feel sleepy or awake. As we are exposed to light in the morning, the SCN sends signals to raise body temperature and produce hormones like cortisol, and delay the release of hormones like melatonin. Melatonin levels rise in the evening as it gets dark and stays elevated throughout the night, promoting sleep.
- Sleep-wake homeostasis – This is the accumulation of hypnogenic (sleep-inducing) substances in the brain which generates a homeostatic sleep drive. During our waking hours, these hypnogenic substances accumulate and increase the pressure to sleep. This pressure is released by the act of sleeping itself, during which the levels of the sleep-regulating substance in the body rapidly declines. The homeostatic sleep drive reminds the body to sleep after a certain time and regulates sleep intensity as well as duration.
Areas of the brain and hormone secretions involved in sleep
Multiple areas of the brain are involved in regulating sleep. Hypothalamus contains the SCN (the biological clock) and in conjunction with the thalamus, it regulates slow-wave sleep. The pons is important for regulating rapid eye movement (REM) sleep.
Sleep is also associated with the secretion and regulation of a number of hormones from several endocrine glands including melatonin, follicle-stimulating hormone (FSH), luteinizing hormone (LH) and growth hormone.
The pineal gland releases melatonin during sleep. It is involved in the regulation of various biological rhythms and the immune system.
During sleep, the pituitary gland secretes both FSH and LH which are important in regulating the reproductive system.
The pituitary gland also secretes growth hormone, during sleep, which plays a role in physical growth and maturation as well as other metabolic processes.
Why we sleep?
There are different theories that explain the function of sleep.
- Repair and Restoration Theory of Sleep – According to this, sleeping is essential for revitalizing and restoring the physiological processes that keep the body and mind healthy and properly functioning. This theory suggests that NREM sleep is important for restoring physiological functions, while REM sleep is essential in restoring mental functions.
- Evolutionary Theory of Sleep – This is also known as the adaptive theory of sleep. It suggests that periods of activity and inactivity evolved as a means of conserving energy. According to this, all species have adapted to sleep during periods of time when wakefulness would be the most hazardous.
- Information Consolidation Theory of Sleep – This suggests that people sleep in order to process information that has been acquired during the day. In addition to processing information from the day prior, this theory also argues that sleep allows the brain to prepare for the day to come. It also suggests that sleep helps cement the things we have learned during the day into long-term memory.
- The Clean-Up Theory of Sleep – This theory suggests that sleep allows the brain to clean itself up. Brain cleans itself of toxins and waste produced during the day while asleep. Brain cells produce waste products during their normal activities. As we sleep, fluid flow through the brain increases. This acts as something of a waste disposal system, cleansing out the brain of these waste products.
Different Stages of Sleep
Sleep can be divided into two different general phases:
- Rapid Eye Movement (REM) sleep – It is characterized by darting movements of the eyes under closed eyelids. Brain waves during REM sleep appear very similar to brain waves during wakefulness.
- Non-REM sleep – It is subdivided into four stages and each stage is differentiated from the other by specific patterns of brain waves.
When we sleep, the first four stages are NREM sleep while the fifth and final stage of sleep is REM sleep.
Each stage of NREM sleep lasts from 5 to 15 minutes.
Stage 1 – This is the first stage of NREM sleep. It is a transitional phase that occurs between wakefulness and sleep. This is the period during which we drift off to sleep. Though our eyes are closed it is quite easy to wake up.
Stage 2 – We are in a state of light sleep. Slow-moving eye rolls discontinue. Our rates of respiration and heartbeat as well as muscle tension and body temperature decreases. Our body is getting ready for deep sleep. In this stage, brain waves continue to slow with specific bursts of rapid activity known as sleep spindles intermixed with sleep structures known as K complexes. Both sleep spindles and K complexes protect the brain from awakening from sleep.
Stage 3 – This is known as deep NREM sleep. This is the most restorative stage of sleep and consists of delta waves or slow waves. It is hard to awaken or arouse someone from Stage 3 sleep. Sleepwalking, sleep talking and night terrors occur during this deepest stage of sleep. During the deep stages of NREM sleep, the body repairs and regrows tissues builds bone and muscle and strengthens the immune system.
As we get older, we sleep more lightly and get less deep sleep. Aging is also linked to shorter time spans of sleep.
Stage 4 – This is where REM sleep starts. It happens 90 minutes after we fall asleep. The first period of REM sleep typically lasts 10 minutes. During this stage, eye movements are rapid, moving from side to side and brain waves are more active than in Stages 2 & 3. It is easy to awake or arouses someone from REM sleep. Being woken from REM sleep, leaves one feeling groggy or overly sleepy. This is a dreaming stage where intense dreams occur, as the brain is very active. Heart rate and breathing quicken.
Babies spend 50% of their sleep in the REM stage.
Adults spend 20% of their sleep in the REM stage.
Sleep debt and sleep deprivation
The cumulative effect of not getting enough sleep over several days, weeks, and months is called sleep debt or sleep deficit. If we have a large sleep debt, it causes mental and physical fatigue. It is of two types:
- Partial sleep deprivation – This occurs when a person sleeps too little for several days or weeks.
- Total sleep deprivation – This occurs when a person is being kept awake for at least 24 hours.
When we have a high sleep debt, it means we need more sleep. So, our brain changes our sleep pattern to include more REM sleep to help our body recover.
Consequences of sleep deprivation are:
- Damages overall work performance
- Reduces concentration levels
- Weakens the immune system
- Raises the level of cortisol (stress hormone) which is linked to brain cell damage
- Impaired reaction time, judgment and vision which leads to safety issues
- Impaired information processing and short-term memory
