Why do we sleep? Scientists understand sleep physiology a great deal (Jacobs, 2009). However, they vary in the precise reason why we sleep. What does it do for our brain and body? Evidence suggests that sleep has a restorative function to reduce and restore our physical energy, flush out toxins in the brain, repair cells and tissues damaged when we’re awake, process and store memories, and turn on our immune system to fight illnesses. Despite differences among scientists on why we sleep, they all agree that quality sleep is necessary to perform at our best.
Stages of sleep
A good night’s sleep begins with closing your eyes and spending a few minutes in a state of relaxed wakefulness. Having thoughts that easily and naturally come and go, your body relaxes, and you drift into Stage 1 sleep.
Stage 1. Stage 1 sleep is a drowsy, relaxed state between waking and sleeping that lasts a few minutes. Your body becomes more relaxed: muscle tension lessens, respiration and heart rate slow, body temperature drops, slow rolling eye movements occur, and your brain generates a slower wave pattern called theta. You might also experience having incomplete thoughts, feeling like you’re daydreaming, or just drifting off if someone were to wake you up.
Stage 2. Stage 2 sleep is a deeper sleep. You feel calmer and disconnected from the world outside of you. Your brain produces wave patterns called K complexes and sleep spindles that represent intermittent attempts by your brain to preserve awareness before you turn off to the world outside of you. Stage 2 is light sleep, from which you might quickly awaken. It lasts about 30 to 45 minutes.
Stages 3 and 4. These stages combined are often referred to as deep sleep. They produce very slow brain wave patterns called delta. Stage 3 is a transitional period between light and the deepest sleep in Stage 4. In deep sleep, physiological activities, like respiration, heart rate, blood pressure, and oxygen consumption, slow down to their lowest levels. It is tough to wake up from a deep sleep. For the most part, your body shuts off from being aware of the world outside of you. Deep sleep can last about 45 minutes before you return to Stage 2 sleep for a few minutes, and then you enter dream sleep, called Rapid Eye Movement (REM) sleep.
REM. In REM sleep, your eyes move around rapidly while you dream. Your brain and body can become very active. For example, heart rate, blood pressure, and breathing rate can increase significantly and become irregular during dreaming. Brain waves and blood flow to the brain can also increase. Brain wave patterns in REM sleep are similar to when awake. However, during REM sleep, your body produces chemicals that paralyze you from acting on your dreams (sleep paralysis).
During a good night’s sleep, you’ll progress through Stages 1 through 4 and REM sleep in about 90 minutes. A good sleeper will complete between 4 and 6 of these 90-minute sleep cycles during the night, spending about
- 5% in Stage 1 sleep
- 50% in Stage 2 sleep
- 20% in Stages 3 and 4 deep sleep
- 25% in REM sleep
Sleep consistently follows this sleep cycle regardless of when you go to bed.
Deep sleep periods are longer at the beginning of the night. They can last up to one hour. In contrast, REM sleep periods are only a few minutes. As you sleep, deep sleep periods get shorter, and REM sleep lasts longer. REM can last up to one hour when reaching your last sleep cycle. So, you get most of your deep sleep during the first half of the night and most of your dream and light sleep in the second half.
Sleep regulators
Processes that affect your sleep include sleep drive, circadian rhythm, and arousal system.
Sleep drive. The strength of your sleep system depends on prior wakefulness. Prior wakefulness is the number of hours that accrue from waking up in the morning to turning off the lights at bedtime. Adenosine is a neuromodulator in your brain that accumulates each hour you’re awake. The earlier you wake up and the longer you’re awake, the more adenosine you build up, which increases the pressure to sleep at night.
Sleeping late, such as staying in bed longer after a poor night’s sleep or on the weekends, reduces your hours of wakefulness and the accumulation of adenosine, which decreases the pressure to sleep at night.
Circadian rhythm. Circadian rhythm is your internal 24-clock that cycles between wakefulness and sleep. It’s also known as your sleep-wake cycle. Your body temperature follows a circadian rhythm. It is lowest in the early morning hours and continues to increase until mid-afternoon when it drops a bit and rises again until it peaks around 6 PM. A few hours later, your body temperature drops until you fall asleep and decreases until it reaches a low again at around 4 AM. In healthy young adults, this temperature variation is about 1½ degrees.
There is also a link between your circadian rhythm, activity level, alertness, and sleepiness. You are most alert and active when your body temperature is at its highest, typically late morning and early evening. You become sleepier and less active as your body temperature decreases at night, with the most pressure to sleep around 3:30 AM. These changes in your body temperature and alertness occur regardless of how well you slept the night before. Even after losing sleep, you’ll feel more alert as your body temperature rises.
The daily cycle of sunlight and darkness affects sleep and body temperature. When sunlight enters your eyes, a naturally occurring hormone in your brain called melatonin decreases, signaling your body temperature to rise. When the sun sets, melatonin levels increase, signaling your body temperature to drop.
Arousal system. The stress response generates a range of physical changes in your body to help you fight or flee from threats or emergencies in the outside world. Changes include a faster heart rate, faster brain waves, increased stress hormones, blood pressure, and muscle tension.
The stress response was designed to help us respond to real stressors, such as predators requiring energy to fight or flee. However, many stressors today can be psychological. They don’t use up the energy generated to fight or flee. Consequently, chronic activation of the stress response can lead to health problems, such as hypertension, headaches, chronic pain, anxiety, or panic disorder.
Over-activation of the stress response during the day can negatively affect sleep. It can cause stress hormones to rise during the day and at night. Daytime stress responses can carry over into sleep. Elevated nocturnal stress hormones strengthen the awake system, dampen the sleep system, and make it harder to fall asleep, stay asleep, and sleep soundly.
Inaccurate or negative thoughts about sleep at bedtime or during the night, such as “If I wake up in the middle of the night, I won’t get back to sleep, I know I won’t sleep well tonight, or sleep is tough for me” can activate the stress response and disrupt your sleep. Other sources of stress activation at bedtime might be thinking about things you need to get done or worrying about things you have no control over. Psychological flexibility training can help with that.
Changes in sleep as you age
Changes in sleep occur as you age:
- Newborns sleep about 16 to 18 hours a day
- Ten-year-olds about 10 hours a day
- Teenagers about 8 eight hours a day
- Middle age, about 7 hours a day
- By 70 years old, about 6½ hours a day, but compensate by taking daytime naps
Sleep quality, particularly among men, can decline in middle age with less deep sleep and more light sleep. Middle-aged people might wake up more often and for longer periods. Some research suggests that older people (around 68) might need less sleep, and their performance is less affected by sleep loss than younger people (around 22).
Body temperature changes with age. For example, daily variations decrease in older people, which might result from less physical activity and exposure to sunlight and cause poor sleep. The body temperature of older people also rises and falls earlier in the day, which often leads to falling asleep and waking up earlier. However, older people tend to adjust favorably to these changes.
Sleep versus tired
People might say “I feel sleepy” or “I feel tired” to mean the same thing. However, the words mean different things. Tired means having low energy, but you don’t necessarily need to sleep. Sleepy means feeling the urge to sleep. Drowsiness is associated with feeling sleepy. Eyelids dropping, head nodding, yawning, or reading the same line in a book several times are some signposts of feeling sleepy. Knowing the difference between sleepy and tired can help you identify when to sleep or rest your body.
Medical and mental health conditions
Sleep apnea is a disorder that causes breathing to stop for seconds to minutes during sleep. Nighttime symptoms include loud snoring, snorting, and gasping for air. Sleep apnea can cause chronic sleep disruptions, leaving people exhausted and sleepy during the day. Other daytime symptoms can include a morning headache or dry mouth. People with sleep apnea might be unaware of their condition unless a partner tells them.
Periodic limb movements (legs or arms twitching, jerking, or even kicking repeatedly from a few minutes to several hours during sleep), restless legs (creeping or crawling sensation in the legs or calves while lying down before bedtime and still awake), delayed phase disorder (can’t fall asleep until late at night, 3 or 4 AM), and hypersomnia (excessive sleep for 10 or more hours) are other conditions that disrupt sleep.
Depression, anxiety, and stressor-related behaviors associated with psychological conditions can disrupt sleep. For example, the rise and fall of body temperature are less in people who experience depression. Flattened body temperature might result from more fatigue and less physical activity, which are common in depression and might exacerbate difficulty sleeping.
Trouble sleeping is common in people with post-traumatic stress injuries. For example, they might avoid sleeping because they’re afraid of having nightmares or bad dreams, remembering what happened, or being unable to respond to potential danger while sleeping.
Medications
Some medications can disrupt sleep, such as asthma medications (e.g., theophylline), over-the-counter decongestants, allergy and cold medicines, steroids (e.g., prednisone), beta-blockers, ADHD medications (e.g., Adderall), and antidepressants (e.g., Wellbutrin).
Ambien, Sonata, Lunesta, and Rozerem (melatonin agonist), are common, non-benzodiazepine insomnia medications. They have fewer side effects but similar risks as their benzodiazepine predecessors, like Ativan. Such risks include daytime sleepiness, cognitive impairment, dizziness, unsteadiness, loss of coordination, and rebound insomnia. Less common but more dangerous risks are sleepwalking or sleep-driving, temporary amnesia or memory lapses, hallucinations, and abuse.
Over-the-counter medicines used for sleep include Tylenol PM, Excedrin PM, Benadryl, and Melatonin. Common anxiolytic or antidepressant medications prescribed off-label for insomnia include Trazodone, Elavil, Remeron, Seroquel, Klonopin, Atarax, and Xanax. Taken in small doses, they produce sedation and drowsiness as a side effect. However, off-label means that these medications were not tested or approved to treat insomnia. Consequently, individuals might be taking drugs that are no more effective than a placebo and might experience significant side effects with long-term use. Physicians may and often use their discretion to prescribe off-label medications.
All prescription medications have side effects, such as a morning hangover, and are only moderately effective. Taking sleeping pills is a short-term solution to insomnia that can lead to long-term problems, such as tolerance and psychological dependence. Taking sleeping pills does not address the causes of insomnia. Improvements in sleep when taking sleeping pills can reinforce a cycle of insomnia and taking drugs that lead to dependency. Insomnia usually returns after individuals stop taking the drugs because the causes of insomnia remain untreated.
Always check with your physician before stopping or tapering sleep medication. Be sure to use sleep medications as instructed by your physician and report any side effects or reactions.
Firearms and sleep
Sleep deprivation, nightmares, or bad dreams might cause feeling confused if you wake up suddenly. So, it can be dangerous to keep a loaded firearm close to you when sleeping. There is a risk of misjudging the situation when you wake up and use the gun when it’s unreasonable.