Sleep is an important part of your daily routine- you spend about one-third of your time doing it. Quality sleep – and getting enough of it at the right times is essential to survival. Without sleep, you can’t form or maintain the pathways in your brain that let you learn and create new memories, and it’s harder to concentrate and respond quickly. Sleep is important to a number of brain functions, including how nerve cells (neurons) communicate with each other. In fact, your brain and body stay remarkably active while you sleep. Sleep plays a housekeeping role that removes toxins in your brain that build up while you are awake.
Everyone needs sleep, but its biological purpose remains a mystery. When we fall asleep, the brain does not merely go offline, as implied by the common phrase “out like a light.” Instead, a series of highly orchestrated events puts the brain to sleep in stages.
Sleep Stages: There are two basic types of sleep: rapid eye movement (REM) sleep and non-REM sleep, which in turn has three different stages. Each is linked to specific brain waves and neuronal activity. You cycle through all stages of non-REM and REM sleep several times during a typical night, with increasingly longer, deeper REM periods occurring toward morning.
- Stage 1: non-REM sleep is the changeover from wakefulness to sleep. During this short period (lasting several minutes) of relatively light sleep, your heartbeat, breathing, and eye movements slow, and your muscles relax with occasional twitches. Your brain waves begin to slow from their daytime wakefulness patterns.
- Stage 2: non-REM sleep is a period of light sleep before you enter deeper sleep. Your heartbeat and breathing slow, and muscles relax even further. Your body temperature drops and eye movements stop. Brain wave activity slows but is marked by brief bursts of electrical activity. You spend more of your repeated sleep cycles in stage 2 sleep than in other sleep stages.
- Stage 3: non-REM sleep is the period of deep sleep that you need to feel refreshed in the morning. It occurs in longer periods during the first half of the night. Your heartbeat and breathing slow to their lowest levels during sleep. Your muscles are relaxed and it may be difficult to awaken you. Brain waves become even slower.
- REM sleep first occurs about 90 minutes after falling asleep. Your eyes move rapidly from side to side behind closed eyelids. Mixed frequency brain wave activity becomes closer to that seen in wakefulness. Your breathing becomes faster and irregular, and your heart rate and blood pressure increase to near waking levels. Most of your dreaming occurs during REM sleep, although some can also occur in non-REM sleep. Your arm and leg muscles become temporarily paralyzed, which prevents you from acting out your dreams. As you age, you sleep less of your time in REM sleep. Memory consolidation most likely requires both non-REM and REM sleep.
Sleep mechanisms: There are two internal biological sleep mechanisms–circadian rhythm and homeostasis–that work together to regulate when you are awake and sleep.
- Circadian rhythms control your timing of sleep and cause you to be sleepy at night and your tendency to wake in the morning without an alarm. Your body’s biological clock, which is based on a roughly 24-hour day, controls most circadian rhythms. One key function of this clock is synchronizing with light cues, ramping up production of the hormone melatonin at night, then switching it off when it senses light. People with total blindness often have trouble sleeping because they are unable to detect and respond to these light cues. They direct a wide variety of functions from daily fluctuations in wakefulness to body temperature, metabolism, and the release of hormones.
- Sleep-wake homeostasis keeps track of your need for sleep. The homeostatic sleep drive reminds the body to sleep after a certain time and regulates sleep intensity. Sleep drive also plays a key role: your body craves sleep, much like it hungers for food. Throughout the day, your desire for sleep builds, and when it reaches a certain point, you need to sleep. This sleep drive gets stronger every hour you are awake and causes you to sleep longer and more deeply after a period of sleep deprivation. A major difference between sleep and hunger: your body can’t force you to eat when you’re hungry, but when you’re tired, it can put you to sleep, even if you’re in a meeting or behind the wheel of a car. When you’re exhausted, your body is even able to engage in microsleep episodes of one or two seconds while your eyes are open. Napping for more than 30 minutes later in the day can throw off your night’s sleep by decreasing your body’s sleep drive.
Dreaming: Everyone dreams. You spend about 2 hours each night dreaming but may not remember most of your dreams. Its exact purpose isn’t known, but dreaming may help you process your emotions. Events from the day often invade your thoughts during sleep, and people suffering from stress or anxiety are more likely to have frightening dreams. Dreams can be experienced in all stages of sleep but usually are most vivid in REM sleep. Some people dream in color, while others only recall dreams in black and white.
Technically sleep starts in the brain areas that produce SWS ( slow-wave sleep). Two groups of cells—the “ventrolateral preoptic nucleus” in the hypothalamus and the “parafacial zone” in the brain stem—are involved in prompting SWS. When these cells switch on, it triggers a loss of consciousness. After SWS, REM sleep begins. This mode is bizarre: a dreamer’s brain becomes highly active while the body’s muscles are paralyzed, and breathing and heart rate become erratic.
Anatomy of Sleep: Several structures within the brain are involved with 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 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 (pons, medulla, midbrain), at the base of the brain, communicates with the hypothalamus to control the transitions between wake and sleep. 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. It sends signals to relax muscles during REM sleep, 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 the 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.
- 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. The release of adenosine from cells in the basal forebrain supports your sleep drive. Caffeine counteracts sleepiness by blocking the actions of adenosine.
So your body rests during sleep, not the brain. The brain remains active, gets recharged, and still controls many body functions. Thus when you sleep, remember – your brain is still at work!