The nature of sleep, including stages of sleep and lifespan changes in sleep Flashcards
Explain sleep
Sleep is a rhythmic process.
a) it is part of the circadian sleep/waking rhythm, partly controlled by endogenous pacemakers (e.g. the SCN), which are adjusted by exogenous pacemakers e.g. light.
b) it has ultradian rhythms, 90 minute (approx) cycles throughout the sleeping night.
What happens during sleep?
There are five stages of sleep: Usually you begin at stage 1 and go through the cycle 2,34,3,2 REM sleep, and then begin the next cycle. Each complete sleep cycle takes from 90 to 110 minutes. A full night’s sleep involves about 4-5 cycles. The length of time in each stage varies according to how far through the night’s sleep the sleeper is, also with their age and how tired they are.
Explain the characteristics of stage 1
The eyes are closed, but the sleeper is easily woken and may feel as if s/he has not slept. Stage 1 may last for five to ten minutes. Many people experience hypnogogic images - floating, flying
Explain the characteristics of stage 2
Stage 2 is a little deeper and the sleeper is a bit harder to awaken. usually lasts about 20 mins. Polysomnograph reading show some bursts of activity called sleep spindles and K-complex waves. These are probably a reposnse to auditory stimuli. The heart rate slows, and body temperature decreases as the body prepares to enter deep sleep.
Explain the characteristics of stage 3 and 4
stage 3 and 4 are deep sleep stages, with stage 4 being deepest. These stages are known as slow-wave sleep (SWS) because of the appearance of low frequency, high amplitude Delta waves in the EEG. These show that brain activity is much reduced. Muscles are very relaxed and the sleeper is hard to waken. Body temperature, blood pressure, and pulse are all at their lowest. Sleep walking, taking and “night terros” occur during this stage. Cycles later in the night have less stage 3 and 4.
Explain the characteristics of REM
REM sleep is charctersised by rapid eye movements. Heart rate and respiration speed up and become erratic. Face and fingers may twitch but the major voluntary muscle groups are paralysed. Intense dreaming occurs during REM sleep. EEG shows that the brain is very active. It is generally thought that REM-associated muscle paralysis is meant to keep the body from acting out dreams. The first period of REM typically lasts about 10 minutes, with each recurring REM stage being longer, meaning that there is more REM later in the night.
Evaluate characteristics of stage and REM sleep
All the above has been found from polysomnograph studies carried out in Sleep Laboratories (e.g. Dement in USA). This means that the participants have been sleeping in unfamiliar surroundings with a variety of recording devices attached to various parts of their bodies and connected with wires to monitoring equipment. It is likely that some participants’ sleep quality may have been affected by this unusual sleeping situation, so we cannot be sure that recordings are typical of sleep that happens at home. The studies also fail to take account of the environmental stimuli that may affect sleep outside of the laboratory, such as partners who snore, traffic noise, early or late sunrise, etc. This means that ecological validity may be low. However polysomnographs are an objective and replicable means of researching sleep giving these studies high internal validity and reliability. Therefore they are highly scientific as is typical of research taking the biological approach. Some of the initial sleep lab research was carried out on small samples often largely made up of men, limiting the generalisability of the early research which did show gender bias. Since then however, the studies have been replicated many times on a wide variety of participants, thus increasing confidence that findings can be generalised to others.
Discuss the amount of sleep we get in our lifespans
Over a typical lifespan, the amount of time we spend each day sleeping declines. Newborns spend from 16-20 hours asleep each day. Between the ages of one and four, total daily sleep time decreases to about 11 or 12 hours. This gradual decline continues throughout childhood. By age ten, sleep amounts of ten hours or less are usual. Adolescents need (ideally) about nine hours of sleep to function at their best. Adults need at least 8 hours.
Discuss the distribution of sleep in our lifespans
i) the newborn’s infant’s sleep is widely distributed around the twenty four hour day (polyphasic sleep). This high need for sleep is probably required for rapid growth.
ii) by sixteen weeks of age clear diurnal pattern emerges, with the main sleep periods at night but also a daytime sleep (biphasic sleep)
iii) By age five, children usually sleep just at night (monophasic sleep)
iv) adolescents show increases in the daytime tendency to fall asleep. Due to changes in circadian rhythms related to development, the preferred times for falling asleep and waking are typically later in adolescents. In extreme cases this can cause real problems and is known as Delayed Sleep Phase syndrome. Many adolescents go to bed relatively later at night because their internal clock prevents them from feeling sleepy until later. However, school start times are usually much earlier than the typical adolescent’s naturally preferred wake time, so many students must wake at unusually early phases of their internal circadian cycle and after too little sleep, leading to chronic sleep deprivation and daytime sleepiness. Exposure to artificial light in the evening (particularly the blue spectrum light from computer screens) fools the internal clock into thinking it is earlier than it actually is, which can make this problem worse. Many adolescents try to catch up on their sleep by sleeping late at the weekends, but this only shifts their internal clock further out of phases with their weekday schedule.
v) among some older people the reverse circadian rhythm disorder may occur, called Advanced Sleep Phase Syndrome; there is excessive sleepiness in the early evening with a resultant tendency to go to bed earlier (advancing the sleep time) and undesired awakening early in the morning. So periods of sleep during the day become more frequent again. This often runs in families.
Discuss developmental changes in REM sleep in our lifespan
The quantity of REM sleep (defined as the proportion of total sleep time) may exceed 50% in the newborn; premature babies have even higher amounts (It is thought that babies in the uterus spend a very large percentage of time in REM). REM sleep amount declines throughout the first year of life. By the time a child is about two years of age, REM is about 20-25% of total sleep, and this remains relatively constant throughout childhood, adolescence and adulthood. The reasons for the high levels of REM in the neonate are unclear, though it has been speculated that REM sleep is important for brain development. Also, babies may alternate between REM and Non-REM sleep every 50-60 minutes, compared with the usual 90 minute pattern in adults. Babies also may go directly into REM when they fall asleep, missing out the first Non-REM cycle at the beginning of the night.
Discuss developmental changes in Non-REM sleep in out lifespan
Stages 3 and 4 of Non-REm sleep (i.e. slow wave sleep) decrease across the lifespan. Children typically have a lot of SWS, during which they are hard to waken by noise and external disruptions. Beginning in early adolescence, there is a gradual decline in SWS and this decline continues throughout adulthood. This decrease in Stages 3 and 4 sleep is generally replaced by Non-REM Stages 1 and 2. By the time the average human reaches age 75, Stage 4, the deepest seep may be almost absent. The cause of the age-related decline in SWS is unknown, but may be an indicator of the ageing of the central nervous system.
Discuss sleep in old age
Old people usually sleep only six to seven hours at night, and their sleep is more fragmented and more easily disrupted by noise. This may be because there is less deep sleep and more shallow sleep which mean that she sleeper is more easily awoken. It maybe that need for sleep is less in older people, or it may be that something prevents old people from getting the sleep they need. Decreased night-time sleep of older people may be partially offset by increased daytime napping (polyphasic sleep), so the 24 hour total sleep time of young and old adults may be quite similar. Some conditions associated with old age affect sleep e.g. Alzheimers is characterised by lethargy; depression, commoner among the older population is associated with sleep disorder. Also many medications to treat illness associated with old age may have drowsiness as a side effect. So the sleep need of older adults remains unclear.
