Sleep

Sleep is as essential to our daily needs as food and water. Although we may feel that sleep simply rests our tired bodies, our brain remains active throughout the night. Sleep plays a critical role in brain as well as physical functioning.

What Happens When We Sleep?

Our internal body clock, called a circadian clock, tells us when we are ready to sleep. There are actually several circadian clocks in the body, found in the brain and other organs. They are triggered by cues such as daylight (we feel alert) and darkness (we feel drowsy). These clocks can also be triggered by artificial bright light or stimulants like caffeine and alcohol that cause us to feel awake even if it is nighttime.

There are several phases of sleep our body experiences. They are classified as REM (rapid eye movement) and non-REM sleep. We cycle repeatedly through these phases about 4-6 times throughout the night, and it is not uncommon to wake up briefly between cycles.

Non-REM sleep

Stage 1. You transition from being awake to a restful state.

Stage 2. You are in a light sleep state. Your breathing, heart rate, and muscle movements slow down. Brain activity also slows, and your body temperature drops.

Stage 3. You are in a deep sleep state. This stage often occurs early in the sleep cycle immediately following light sleep. Your heart rate and breathing are the slowest during this phase, and you are not easily awakened. Events of the day are processed and stored in your memory. A lack of deep sleep can leave one feeling tired in the morning even if achieving an adequate duration of sleep.

REM sleep

During REM, your pupils twitch and move quickly from side to side underneath closed eyelids. Brain activity rises as you breathe faster and your heart rate increases. It is the phase of sleep when dreams are most common, and certain nerves signal your limbs to become temporarily paralyzed so you do not act out the dream. REM tends to occur later at night and into early morning. Memory is processed and stored during REM sleep.

Why do we dream?

Dreams occur during the REM sleep stage and may include events or thoughts you experienced recently. Dreams may occur to help process emotions caused by those events. People report dreaming in vivid color as well as in black and white. Sometimes even a wild crazy dream can be forgotten by the time we awaken in the morning.

Hormones that Regulate Sleep Cycles

There are various neurotransmitters and hormones released by the brain that send signals to promote sleep or wakefulness. [1] Many of these chemicals are stimulated by light or darkness.

• GABA is a neurotransmitter that decreases nerve cell activity, playing a major role in allowing the body to sleep.
• Adenosine is another neurotransmitter that gradually accumulates in the brain during the day, and at high concentrations makes us sleepy at night. Caffeine in coffee and other beverages can keep us awake as it blocks brain receptors for adenosine.
• Melatonin is a hormone released by the brain when it is dark. It travels to cells to tell the body to sleep. Sunlight or exposure to light inhibits the production of melatonin and increases the release of cortisol, which awakens us. If we are exposed to too much artificial light (such as the blue light emitted from smartphones or televisions) late at night, less melatonin may be released making it harder to fall asleep.
• Serotonin, the body’s “feel-good” chemical, is a neurotransmitter associated with both sleep and being awake. The brain releases this chemical during daylight but also uses it to form melatonin at night.
• Hormones that counteract sleep include norepinephrine, adrenaline, histamine, and cortisol. These are secreted in response to stress and cause the body to be awake and alert. If one experiences prolonged or chronic stress, the body releases adrenocorticotropic hormone (ACTH), which in turn releases cortisol. Levels of ACTH tend to be higher in people who have insomnia. 

Immediate Effects of Sleep Deprivation

About one-third of American adults do not get enough sleep each night, according to the Centers for Disease Control and Prevention. [2,3] Short sleep duration in adults is defined as less than 7 hours of sleep in 24 hours. About 40% of adults report unintentionally falling asleep during the day at least once a month, and up to 70 million Americans have chronic sleep problems. Because of the public health burden of poor sleep health, achieving sufficient sleep in children and adults was included as a goal in the Healthy People 2020 goals. [4]

Sleep helps to process your thoughts from the day as well as store memories, so a lack of good-quality sleep can lead to difficulty focusing and thinking clearly. You may feel tired, irritable, or anxious during the day. Performance at work or school may suffer. Your reaction time may be slowed, increasing the risk of driving accidents.

In children, insufficient sleep can lead to attention and behavior problems or hyperactivity. In the elderly, lack of sleep may decrease focus and attention, leading to a greater risk of falls, bone fractures, and car accidents.

There are several reasons people may get insufficient sleep:

• Poor sleep habits (watching television or using screens late at night, drinking caffeinated or alcoholic beverages at night, not following a regular sleep schedule).
• Your sleep environment is too noisy, too light or otherwise not conducive to sleep.
• You attempt to sleep outside of the body’s natural circadian clock (working an overnight shift and trying to make up for sleep during the day).
• You have a sleep disorder, such as sleep apnea, insomnia, or periodic limb movements that reduces deep or REM sleep or causes frequent awakenings.
• You have a medical condition such as heart, lung or kidney disease, or chronic pain, which causes frequent awakenings.

Sleep Deficiency and Disease Risk

If you experience continued sleep deprivation, you will develop a condition called sleep deficiency. This is a state in which you cannot make up the many lost hours of sleep. Sleep deficiency increases the risk of obesity, diabetes, cardiovascular disease, depression, and even early death.