To understand the physiology of sleep, we must understand the physiology of the brain, because the most pronounced physiological changes in sleep occur in the brain.
The brain uses significantly less energy during sleep than it does when awake, especially during non-REM sleep. In areas with reduced activity, the brain restores its supply of ATP (adenosine triphosphate) – the molecule used for short-term storage and transport of energy. In quiet waking, the brain is responsible for 20% of the body’s energy use, thus this reduction has a noticeable effect on overall energy consumption.
Sleep increases the sensory threshold, that means that sleeping persons perceive fewer stimuli, but can generally still respond to loud noises and other salient sensory events.
During slow-wave sleep humans secrete bursts of growth hormone.
All sleep, even during the day, is associated with the secretion of prolactin.
Key physiological methods for monitoring and measuring changes during sleep include:
– EEG – electroencephalography of brain waves,
– EOG electrooculography of eye movements, and
– EMG electromyography of skeletal muscle activity.
Simultaneous collection of these measurements is called (PSG) polysomnography, type of sleep study which can be performed in a specialized sleep laboratory.
Sleep researchers also use:
– simplified EKG – electrocardiography for cardiac activity and
– actigraphy for motor movements.
BRAIN WAVES IN SLEEP
The electrical activity seen on an EEG represents brain waves. The amplitude of EEG waves at a particular frequency corresponds to various points in the sleep-wake cycle, such as being asleep, being awake, or falling asleep.
Alpha, beta, theta, gamma, and delta waves are all seen in the different stages of sleep and each waveform maintains a different frequency and amplitude:
– ALPHA WAVES are seen when a person is in a resting state, but is still fully conscious. Their eyes may be closed and all of their body is resting and relatively still, where the body is starting to slow down.
– BETA WAVES take over alpha waves when a person is at attention, as they might be completing a task or concentrating on something. Beta waves consist of the highest of frequencies and the lowest of amplitude, and occur when a person is fully alert.
– GAMMA WAVES are seen when a person is highly focused on a task or using all their concentration.
– THETA WAVES occur during the period of a person being awake, and they continue to transition into Stage 1 of sleep and in stage 2.
– DELTA WAVES are seen in stages 3 and 4 of sleep when a person is in their deepest of sleep.
NON-REM AND REM SLEEP
Sleep is divided into two broad types: non-rapid eye movement (non-REM or NREM) sleep and rapid eye movement (REM) sleep. Non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states.
NON-REM sleep occurs first and after a transitional period is called slow-wave sleep or deep sleep. During this phase, body temperature and heart rate fall, and the brain uses less energy.
REM SLEEP, also known as paradoxical sleep, represents a smaller portion of total sleep time. It is the main occasion for dreams (or nightmares), and is associated with desynchronized and fast brain waves, eye movements, loss of muscle tone, and suspension of homeostasis.
The SLEEP CYCLE of alternate NREM and REM sleep takes an average of 90 minutes, occurring 4–6 times in a good night’s sleep.
The American Academy of Sleep Medicine (AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep.
The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM.
REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep. There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening.
AWAKENING
Awakening can mean the end of sleep, or simply a moment to survey the environment and re-adjust body position before falling back asleep. Sleepers typically awaken soon after the end of a REM phase or sometimes in the middle of REM. Internal circadian indicators, along with a successful reduction of homeostatic sleep need, typically bring about awakening and the end of the sleep cycle. Awakening involves heightened electrical activation in the brain, beginning with the thalamus and spreading throughout the cortex.
On a typical night of sleep, there is not much time that is spent in the waking state. In various sleep studies that have been conducted using the electroencephalography, it has been found that females are awake for 0-1% during their nightly sleep while males are awake for 0-2% during that time. In adults, wakefulness increases, especially in later cycles. One study found 3% awake time in the first ninety-minute sleep cycle, 8% in the second, 10% in the third, 12% in the fourth, and 13–14% in the fifth. Most of this awake time occurred shortly after REM sleep.
Today, many humans wake up with an alarm clock, however, people can also reliably wake themselves up at a specific time with no need for an alarm. Many sleep quite differently on workdays versus days off, a pattern which can lead to chronic circadian desynchronization. Many people regularly look at television and other screens before going to bed, a factor which may exacerbate disruption of the circadian cycle. Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying sleep inertia.
Determinants of alertness after waking up include quantity/quality of the sleep, physical activity the day prior, a carbohydrate-rich breakfast, and a low blood glucose response to it.
BRAIN STRUCTURES INVOLVED WITH SLEEP
Different brain structures are involved during the sleep.
THE HYPOTHALAMUS, a peanut-sized structure deep inside the brain, contains groups of nerve cells that act as control centers affecting sleep and arousal. Within the hypothalamus is the suprachiasmatic nucleus (SCN) – clusters of thousands of cells that receive information about light exposure directly from the eyes and control your behavioral rhythm. Some people with damage to the SCN sleep erratically throughout the day because they are not able to match their circadian rhythms with the light-dark cycle. Most blind people maintain some ability to sense light and are able to modify their sleep/wake cycle.
THE BRAIN STEM, at the base of the brain, communicates with the hypothalamus to control the transitions between wake and sleep. The brain stem includes structures called the pons, medulla, and midbrain. Sleep-promoting cells within the hypothalamus and the brain stem produce a brain chemical called GABA, which acts to reduce the activity of arousal centers in the hypothalamus and the brain stem. The brain stem, especially the pons and medulla, also plays a special role in REM sleep; it sends signals to relax muscles essential for body posture and limb movements, so that we don’t act out our dreams.
THE THALAMUS acts as a relay for information from the senses to the cerebral cortex – the covering of the brain that interprets and processes information from short- to long-term memory. During most stages of sleep, the thalamus becomes quiet, letting you tune out the external world. But during REM sleep, the thalamus is active, sending the cortex images, sounds, and other sensations that fill our dreams.
THE PINEAL GLAND, located within the brain’s two hemispheres, receives signals from the SCN and increases production of the hormone melatonin, which helps put you to sleep once the lights go down. People who have lost their sight and cannot coordinate their natural wake-sleep cycle using natural light can stabilize their sleep patterns by taking small amounts of melatonin at the same time each day. Scientists believe that peaks and valleys of melatonin over time are important for matching the body’s circadian rhythm to the external cycle of light and darkness.
THE BASAL FOREBRAIN, near the front and bottom of the brain, also promotes sleep and wakefulness, while part of the midbrain acts as an arousal system. Release of adenosine (a chemical by-product of cellular energy consumption) from cells in the basal forebrain and probably other regions supports your sleep drive. Caffeine counteracts sleepiness by blocking the actions of adenosine.
THE AMYGDALA, an almond-shaped structure involved in processing emotions, becomes increasingly active during REM sleep.
I hope that the physiology of sleep is now more clearly understood. If not, we are open to explore with you additional questions that you have.