We spend around a third of our lives asleep, but the average amount of sleep Americans get each night has fallen in recent years. This has been linked to factors such as shift-work, multiple jobs and spending time watching television and using the Internet.1
The importance of getting sufficient, good quality sleep is increasingly recognized, but the scientific study and understanding of sleep is a relatively recent development.1 Sleep has become something of a national concern, in fact, with insufficient sleep now viewed by some as a public health problem.2
Contents of this article:
There are introductions at the end of some sections to recent developments that have been covered by MNT's news stories. Also look out for links to information about related conditions.
Here are some key points about sleep. More details and supporting information are in the body of this article.
- The need for sleep varies considerably as people age.
- Individuals of the same age may have different sleep needs.
- Pregnant women often feel sleepy during the day, especially during the first trimester.
- Sleep has an effect on our performance, mood and general health.
- Sleep cycles through two types - rapid eye movement (REM), when dreaming occurs, and non-REM sleep.
- Sleep deprivation has acute and long-term adverse effects on health, including an increased risk of premature death.
- Most tips for a good night's sleep are based on good routines.
- Many sleep disorders are characterized by excessive daytime sleepiness, difficulty getting to sleep or maintaining sleep, or abnormal events during sleep.
How much sleep do I need?
The US National Institutes of Health (NIH) give the following guidance for how many hours of sleep are needed each night according to age group:1,3-5
- Adults (between the ages of 18 and 64 years) - from 7-9 hours
- Pregnant women - additional daytime or nighttime hours
- Older adults (over the age of 64 years) - from 7-8 hours.
As people get older, the amount of sleep they require typically declines.
The American Thoracic Society agrees with this guidance for healthy sleep in adults.
The amount of sleep needed varies between individual, however, and can be gauged against how different amounts of sleep affect daytime feelings and productivity for each person. Feeling sleepy or depending on caffeine during the day, for example, may signal insufficient or poor quality sleep.6
As we grow older, the structure of sleep shows a continuous, considerable change. These changes in the "sleep architecture" affect:1
- How sleep is initiated and maintained
- The relative time spent in each stage of sleep
- Sleep efficiency (success in starting and staying asleep).
In general, overall amount of sleep and sleep efficiency both decline with age.1
As we age, we tend to wake earlier and a tendency toward earlier bedtimes develops. People between the ages of 65 and 75 years, for example, typically wake up 1.33 hours earlier and go to bed 1.07 hours earlier than young adults (between the ages of 20-30 years).1
Dramatic decreases in melatonin synthesis in older adults has been linked to sleep disorders and a range of adverse health conditions. Melatonin is the neurohormone produced in response to diminishing light levels at dusk, with levels dropping in the early morning before we wake. Shift work, overseas travel, aging and other facts can affect melatonin synthesis, which can then disrupt sleep patterns and sleep quality.
How much sleep does my child need?
The NIH offers the following guidelines for sleep needed by infants, children and teenagers:1,3-5
- Newborn babies (up to 3 months of age) - from 14 to 18 hours
- Young infants (between the ages of 4 and 11 months) - from 12 to 16 hours
- Toddlers (1- and 2-year-olds) - from 11 to 14 hours
- Preschool children (between 3 and 5 years) - from 10 to 13 hours
- Schoolchildren (between 6 and 13 years) - from 9 to 11 hours
- Teenagers (between 14 and 17 years) - from 8 to 10 hours.
Newborn babies do not have circadian rhythms. The need to sleep during the night rather than the day as part of a 24-hour cycle does not develop until the age of 2 or 3 months. Very young babies do not have long continuous episodes of sleep; instead, they sleep for 16-18 hours a day for short periods of between 2.5 and 4 hours.1
By 12 months of age, infants develop sleep patterns involving less sleep overall which is concentrated more around the nighttime. They also lose a feature of young babies' sleep known as active sleep in which there is a lot of body movement. Instead, muscle paralysis and atonia takes place during REM sleep.1
Physiological needs, cultural environment and social changes such as reduced daytime napping and school routines mean that the amount of sleep children get progressively decreases into adolescence.1
Adolescent children - determined by puberty rather than age in years - need as much as 10 hours of sleep every night, but tend to get under 8 hours. This optimal sleep duration for adolescents is based on research about alertness, sleep-wake cycles, hormones, and circadian rhythms.1
Sleep during pregnancy
Pregnant women tend to need more sleep than other women and are advised to plan for changes to their sleep during their first trimester of pregnancy.
Pregnant women often experience considerable daytime sleepiness, which can also be felt in the first few months after giving birth (the postpartum months).1
In addition, pregnant women are more likely to experience restless legs syndrome (RLS), which can affect the quality of sleep.1
The National Sleep Foundation recommend that first-time pregnant women plan for changes to sleep in their first trimester of pregnancy - the first 3 months. The US not-for-profit organization cites the effects of the hormone progesterone on sleep and gives tips for early pregnancy such as:7
- Sleep whenever and wherever possible
- Take daytime naps when needed
- Sleep on the left side to improve the flow of blood and nutrients to the fetus
- Drink less before bed and, if awoken, go to the toilet at night
- Reduce sleep disturbance by avoiding putting on bright lights.
Recent developments on sleep from MNT news
Evidence from hunter-gatherer communities suggests it could. The findings were published in the journal Cell Press in October 2015.
A study published in Trends in Neurosciences in September 2015 found that slow-wave sleep (deep sleep) helps with the storage of information about pathogens.
Research published in The Journal of Sexual Medicine in March 2015 tested the relationship between sleep duration and quality and women's sex lives for the first time.
Why do we sleep?
Sleep is needed for us to feel rested, but a fuller scientific understanding of sleep is relatively recent. These new insights follow developments in the fields of somnology (the scientific study of sleep physiology) in the 1970s and sleep medicine (diagnosis and treatment of disordered sleep) since the 2000s.1
While the function of sleep is not fully understood, almost all animals need it, and the consequences of sleep loss in humans can be serious.1
The function of sleep is often explained in terms of the effects of reduced sleep on the next day's functioning. By knowing the consequences of lost sleep, it is possible to conclude that it is necessary for:3-5
- General health.
In terms of performance, sleep contributes to the proper functioning of the nervous system, including cognitive abilities and emotional health. Sleep deprivation can decrease alertness and reduce response times, with some studies showing that 20-25 hours without sleep is akin to having a blood alcohol level over the limit for driving and operating heavy machinery.3-5,8
Brain imaging has shown that pathways for memory and learning are active during sleep. The REM stage of sleep stimulates learning regions. We need sleep for clear thinking, normal reactions and the creation of memories.3-5
Emotional and social functioning may be dependent on good sleep, and mood is affected by deprivation - perhaps even leading to a greater risk of depression.3-5
The need for sleep to maintain good health is shown starkly by the reduced survival of mammals that are deprived of it. The normal lifespan of a rat - 2 to 3 years - falls to 5 weeks when REM sleep is lost, and 3 weeks when all stages of sleep are lost.5
In addition to producing growth hormone essential to childhood development, sleep enables the secretion of other hormones in adults, too, which help:4
- Build muscle
- Fight illnesses
- Repair damage.
High blood pressure, heart disease and other medical conditions may be more likely if sleep is poor in quantity or quality. Sleep also appears to have benefits for energy use, with poor sleep also raising the risks of weight gain, obesity, diabetes and poorer dietary choices.3
Ironically, people who are obese or overweight are more likely to suffer from sleep apnea, which disrupts sleep and makes it harder to lose weight.
What happens during sleep?
Sleep scientists have separated sleep into two types through which we cycle alternately:1
- Rapid eye movement (REM) sleep
- Non-rapid eye movement (NREM) sleep.
While these two types of sleep have been described and shown to be disturbed in sleep disorders, the reason for the alternation is not understood.1
REM sleep accounts for 20-25% of normal sleep duration; NREM makes up the remainder. As this table from the Institute of Medicine shows, there are physiological differences between the two stages of sleep. During NREM sleep, there is a decrease in brain activity and blood pressure in motor and sensory areas; these increase during REM sleep.1
REM sleep is associated with dreaming, and the temporary loss of muscle tone and reflexes during this stage is thought to prevent the "acting out" of dreams or nightmares.1
Some researchers have suggested that the atonia associated with REM sleep may serve the purpose of decompressing the intervertebral discs by enabling the relaxation of muscles and nerves that normally closely regulate spinal rigidity.9
A rebound sleep effect has been observed, whereby people who have REM sleep disrupted go on to have a greater proportion of REM sleep at the next opportunity. Older adults who get less sleep on average than younger adults have also been found to get around the same overall amount of REM sleep.
Why we dream during our sleep is not known, but we do it for over 2 hours each night. Neuroscientists have, however, mapped the brain's activity during REM sleep and found that it stimulates the brain regions used in learning.5
Effects of sleep deprivation
Adults are considered to have sleep deprivation when they get less than the average need of 7-8 hours sleep a night, and insufficient sleep in the US has been described as a public health problem.1,2
Estimates put the number of Americans chronically suffering from a disorder of sleep at 50-70 million.1,2
Less prevalent but potentially more serious are the consequences of impaired judgment due to sleep loss, such as road traffic accidents.1
Learn more about sleep deprivation, including its symptoms, complications and treatment.
How to get a good night's sleep
Napping during the day can have a negative impact on bedtime sleepiness.
Sleep hygiene is a term used to describe a routine that promotes good sleep. Sleep hygiene tips have been drawn up separately by the American Academy of Sleep Medicine, the American Sleep Association, the NIH and the National Sleep Foundation - all recommend the following practical measures:3,8-10
- Keep to the same sleeping and waking times - form a routine bedtime and wake schedule and keep it consistent across the week, including weekends
- Set a bedtime for when you will feel sleepy and that will leave at least 7 hours for sleeping
- Avoid daytime naps that reduce bedtime sleepiness
- Before going to bed, establish a routine that promotes sleep, avoiding alcohol, caffeine, smoking and having a lot to eat or drink
- A regular winding down ritual may be helpful, which could include a bath, reading or meditation, and would avoid, for example, heated discussions before bed
- Do not watch TV, browse the Internet, and so on, before bedtime, and do not do these activities in bed
- Keep the bedroom quiet, dark and comfortable - a comfortable temperature for sleeping is slightly cooler than room temperature
- Do not stay in bed awake after more than 20 minutes of trying to get to sleep - reduce the association with an inability to sleep by going to another room and try reading in a seat until feeling sleepy
- Exercise may be beneficial for sleep - vigorous exercise during the day and relaxing exercise, such as yoga, before bed can both help.
Many of these sleep hygiene practices help the body to properly regulate melatonin synthesis. Melatonin is the chemical produced by the pituitary gland as natural light levels drop at dusk, prompting the body to prepare itself for sleep.
Use of artificial light, including the light emitted by televisions, phones and computers, can trick the brain into thinking it is still daylight, thereby inhibiting melatonin synthesis and delaying sleep. People who are unable to entirely cut out screen-time after dusk may find it helpful to use specific software on screens to filter out blue light in the evening.
The National Sleep Foundation have identified groups of sleeping patterns through surveys; there are supposedly five types of sleep personality, including the "healthy, lively larks" and the "sleep-savvy seniors."
Suboptimal sleep - whether too little, too much or of poor quality - results in raised levels of calcium in the coronary arteries and arterial stiffness, according to a study published in September 2015 in the journal Arteriosclerosis, Thrombosis and Vascular Biology.
Another study published in 2015 found sleep links to heart disease, raising calls for poor sleep to be considered as a risk factor for cardiovascular disease in prevention guidelines.
A study published in the journal Sleep in August 2015 suggested that less than 5 hours of sleep every night for a week increases the risk of developing a cold by 4.5 times.
Sleep medicine specialists have identified that there are around 90 distinct sleep disorders and that most have one of the following features:1
- Excessive daytime sleepiness
- Difficulty getting to sleep or staying asleep
- Abnormal movements, behaviors or sensations during sleep.
Separate medical conditions can also adversely affect sleep. Pain, infection, chronic obstructive pulmonary disease and peptic ulcer disease are common examples.1,11
Circadian rhythm sleep disorders include the effects of jet lag and shift work.1
Parasomnias are unpleasant or undesirable sleep behaviors or experiences, including disorders of arousal in which there may be disoriented sleepwalking, shrieking or flailing limbs.
A number of other distinct sleep disorders are covered in more detail by MNT - follow these links to their respective Knowledge Center pages:1