Sleep Stages: Who Enters Rem Sleep First?

Sleep is a complex and mysterious process that humans spend about a third of their lives doing. On average, a good night's sleep should constitute about 7-9 hours, but this differs across age groups. A typical night of sleep consists of four to six sleep cycles, each lasting about 90-120 minutes. Each cycle includes three stages of non-rapid eye movement (NREM) sleep and one stage of rapid eye movement (REM) sleep. NREM sleep is further divided into three stages, with each stage representing a continuum of relative depth. The first stage of NREM sleep is the lightest, while the third stage is the deepest. As people age, they tend to spend less time in the deeper stages of NREM sleep and more time in the second stage. REM sleep, on the other hand, is associated with dreaming and increased brain activity. While the ideal amount of REM sleep is not known, it is believed to be important for learning, memory, and emotional processing.

The elderly

Sleep patterns change as we age, and the elderly experience a variety of sleep alterations. Elderly people may struggle to obtain the recommended seven to nine hours of sleep in one block, despite still requiring up to eight hours. This is due to a decrease in slow-wave sleep (SWS), the deepest stage of sleep, which is responsible for tissue repair and strengthening the immune system. Older adults experience shorter periods of SWS and fewer of them, resulting in lighter and more fragmented sleep with brief arousals or longer awakenings throughout the night.

The sleep-wake cycle is regulated by the interaction of two major processes: one that promotes sleep and one that maintains wakefulness. With age, the window during which the internal clock enables sleep narrows, which may explain why older people tend to wake up early in the morning and fall asleep earlier in the evening. This change in the circadian rhythm is also associated with a reduced ability to adjust to phase-shifting, such as shift work or jet lag.

Age-related changes in sleep are influenced by physiological, hormonal, and psychosocial factors. For example, the decline in slow-wave sleep may be related to a decrease in growth hormone secretion, which peaks during adolescence and gradually declines with age. Additionally, older adults may nap more frequently due to biological changes and lifestyle alterations that come with aging, such as reduced physical activity and social engagement.

Furthermore, elderly individuals are more likely to experience sleep disturbances due to medical comorbidities, psychiatric conditions, primary sleep disorders, and changes in their environment, social engagement, and lifestyle. These factors contribute to the high prevalence of sleep problems in older adults, with up to 50-60% reporting poor sleep quality. However, it is important to note that healthy older adults without any comorbidities are less likely to report sleep issues and tend to accept some sleep alterations as a normal part of aging.

Newborns and infants

The sleep cycles of newborns and infants are divided into two main stages: REM sleep and non-REM sleep. REM sleep, or "active sleep," is characterised by rapid eye movements, increased brain activity, and vivid dreaming. During this stage, newborns may make small movements, such as twitching fingers and limbs, and their breathing may speed up. Non-REM sleep, or "quiet sleep," consists of four substages, with the third and fourth substages being the deepest and most restorative sleep.

As babies grow older, the proportion of REM sleep decreases, and their sleep cycles become more similar to those of adults. By the time they are six months old, babies sleep for an average of 13 hours a day, in larger blocks of time.

Toddlers and children

Sleep is divided into three stages of NREM sleep and one stage of REM sleep. NREM sleep is further divided into three stages:

• Stage 1: Light sleep, where the person is asleep but their skeletal muscle tone and breathing are the same as when they are awake.
• Stage 2: Deeper sleep, where the heart rate and breathing slow down and the body temperature drops.
• Stage 3: Deepest sleep, where the body repairs and regrows tissues, builds bone and muscle, and strengthens the immune system.

REM sleep, on the other hand, is associated with dreaming and accounts for about 25% of total sleep time in children. During REM sleep, the eyes move rapidly behind closed eyelids, and the brain activity is similar to when a person is awake.

The amount of sleep needed varies depending on age. Toddlers between the ages of one and five require between 10 and 14 hours of sleep, including naptime. Children between the ages of six and twelve need slightly less sleep, ranging from nine to twelve hours.

It is important to note that these sleep amounts are not universal and can vary from person to person. Some children may need more or less sleep depending on various factors, such as genetics or health status.

Adolescents

During adolescence, the amount of sleep individuals need varies, but on average, teenagers require around 8 to 10 hours of sleep per night. The decline in NREM sleep during this period is attributed to the decrease in slow-wave sleep (SWS), which includes stages 3 and 4 of NREM sleep. This reduction in SWS is linked to the decrease in delta power, a measure of high-amplitude delta waves associated with intense NREM sleep. The reduction in delta power is a result of synaptic elimination during adolescence, leading to a less intense and more mature brain activity pattern.

While NREM sleep decreases, adolescents experience a slight increase in REM sleep duration. This increase becomes more pronounced during extended sleep, when individuals are allowed to sleep longer. The reasons for this are not fully understood but may be related to the brain's need for REM sleep to support its maturation and the potential impact of changing circadian rhythms.

The changes in sleep architecture during adolescence have important implications for overall health and well-being. NREM sleep, particularly the deeper stages, is crucial for tissue repair, immune system strengthening, and the release of essential hormones. On the other hand, REM sleep plays a vital role in learning, memory, and emotional processing. Therefore, adequate amounts of both NREM and REM sleep are necessary for adolescents' physical and mental development.

Furthermore, the quality of sleep during adolescence is essential. Sleep deprivation or disrupted sleep patterns can lead to physical and mental health issues, cognitive impairment, and an increased risk of cardiovascular disease and metabolic conditions like Type 2 diabetes. Establishing healthy sleep habits and maintaining a consistent sleep schedule are crucial for adolescents to get the restorative sleep they need.

Adults

Sleep is a complex and mysterious body process that humans spend about a third of their lives doing. On average, adults need seven to nine hours of sleep per night, with about 25% of this being deep sleep. However, the amount of sleep an individual needs can vary, and this can be influenced by factors such as genetics, health status, and personal circumstances.

Sleep is divided into two types: non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep is further divided into four stages, with the first three being progressively deeper stages of sleep. During the deepest NREM sleep, the body repairs and regrows tissues, builds bone and muscle, and strengthens the immune system. REM sleep, on the other hand, is associated with dreaming and memory consolidation.

As people age, they tend to sleep more lightly and get less deep sleep. Older adults also tend to wake up and go to bed earlier than younger adults. Men are more likely to spend more time in the first stage of NREM sleep and experience more awakenings throughout the night, while women are more likely to maintain deep sleep for longer and complain of difficulty falling asleep.

The quality and duration of sleep can be affected by various factors, including depression, aging, traumatic brain injuries, medications, and circadian rhythm disorders. Sleep is important for both physical and mental health, and lack of sleep can lead to neurological dysfunction and an increased risk of developing obesity, diabetes, and cardiovascular disease.

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