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 paralysed 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.

How Much Sleep Do We Need?

Sleep needs change as we age, with the average person generally requiring less sleep at older ages. However, specific sleep amounts vary by individual. According to the National Sleep Foundation and American Academy of Sleep Medicine (AASM), newborns need the most sleep, at 14-17 hours a day, followed by infants at 12-16 hours a day including naps. Toddlers need about 10-14 hours a day. Preteens and teenagers need about 8-12 hours a night, and adults about 7-8 hours a day. [29] A consensus by the AASM and Sleep Research Society recommends that adults should sleep 7 or more hours a night to promote optimal health. 

Despite these general recommendations on sleep duration, individual differences in sleep requirements exist. In most epidemiologic studies, increased risk of adverse health outcomes has been observed among those who reported sleeping 5 hours or less per day, and 9 hours or more per day. Thus, a range of sleep hours (more than 5 and less than 9) is considered appropriate for most healthy adults.

Other factors such as quality of sleep are important, because just meeting the total recommended sleep hours may not be enough if one wakes up frequently in the night. A common belief is that lost sleep from a late night out or studying can be recovered by “sleeping in” another day or taking naps. However, both of these methods disrupt the body’s circadian rhythms and may deprive the body of deeper sleep stages. In fact, increased variability in how much sleep we get from night to night is associated with an increased risk of developing metabolic and heart diseases. It is important to respond, whenever possible, to the body’s natural signals of sleepiness.

Herbal supplements

Two popular herbal supplements, melatonin and valerian, are used as sleep aids. I have shown melatonin to quicken time to sleep and have modest benefits on sleep duration and quality, but can cause daytime drowsiness. It is well tolerated in adults with few reported adverse events in doses up to 10 mg. 

Valerian contains small amounts of GABA, a sleep-promoting neurotransmitter, and some studies have shown that valerian can improve sleep. 

Does exercising at night disrupt sleep?

A solid body of research confirms that both aerobic and resistance exercises  improve sleep, with reported improvements comparable to the effects of pharmacological treatments for insomnia if exercises are performed regularly. A review of 13 studies found that regular resistance exercises (using resistance bands, strength machines) performed 2-3 times a week for an average of one hour improved sleep quality. It also found that resistance exercises decreased anxiety and depression, which could be a factor in improving sleep. However, it is not recommended to do vigorous exercises such as running or interval training within one hour of bedtime. A review of 23 studies found that healthy adults who performed high-intensity exercise too soon before bed had difficulty falling asleep and experienced poorer sleep quality. Yet, this study and others did not find that performing more moderate exercises or exercising vigorously further away from bedtime caused sleep disturbances, and in fact helped people to experience increased amounts of deeper sleep.