Stages of sleep (NREM)
Awake (Beta waves) - while awaek and alert the EEG shows waves that are small, irregular and frequent.
Just prior to sleep (Alpha) - Once in bed, the person becomes relaxed and drowsy. During this time some 'dream like' experiences begin to occur.
Stage one (Theta) - Brain wave activity slows down even more to become theta waves. Heart rate slows and muscles relax.May experience the sensation of falling followed by a sudden jerk. It is easy to wake someone from this stage and they may not even feel as if they have been asleep.
Stage 2 (Theta) - Heart rate, BP and body temp drop and eyes roll slowly from side to side. The EEG shows sleep spindles which are high frequency spindle shaped burst of activity lasting about 1-2 seconds.
Stage 3 (20-30% delta) - Sleep spindles disappear. The individual is now in a deep sleep from which they cannot easily be woken.
Stage 4 (delta) - Sleep is very deep. It is very difficult to be woken. During this stage, things like night terrors and sleep walking are most likely to occur.
REM (rapid eye movement) - This stage is characterised by rapid eye movements during which the eyes movve rapidly back and forth so vigorously that they can actually be observed through closed eye lids. Strangely, the EEG shows Beta waves similar to those of being awake. REM sleep is sometimes called paradoxical sleep because although the EEG shows that same pattern as if awake, the body becomes virtually paralysed.
It is more difficult to wake someone from REM sleep than from any other stage. The heart rate increases, blood pressure rises and breathing becomes faster and more irregular.
Life Span Changes in Sleep
Sleeping patterns vary by age both qualitatively (time spent in dfferent stages of sleep) and quantitatively (how many hours of sleep a night). Endogenous bioligcal rhythms are synchronised with signals from the outside world.
- Newborns sleep on average 16 to 18 hours a day. (divided equally between REM and NREM sleep).
- By age 1, children usually sleep 13-14 hours, with the entire sleep stage cycle occuring every 45-60 minutes.
- Between 5 and 12, total sleep is 9-10 hours and they sleep deeply.
- There are few significant changes in sleep patterns between the teenage years and early adulthood although social factors such as work pressure etc.
- By middle age, there are noticeable changes in sleep patterns. Sleep duration reduces to 7 hours and by 60, stage 4 of sleep virtually disappears. There is a corresoponding increase in lighter stages of sleep.
- Beyond 60, sleep is characterisedby frequent interruptions as older peopple spend more time in lighter stages of sleep and therefore may be more easily woken up by external noises.
Research study: Ohayon et al (2004)
Ohayon et al (2004) performed a meta review using 65 studies of sleeping patterns that between them covered the age range of 5-102. They identified a number of significant trends:-
- Total sleep time decreased steadily from about 470 minutes at age 5 to 370 minutes at about 70.
- The percentage of stage 1 increases from 5.8% at age 5 to 6.8% at age 70.
- The % of stage 2 NREM shows a similar increase over the lifespan.
- The most dramatic finding was a decrease in the % of deep NREM (Stage 3+4) from 24% at age 5 to only about 9% at age 70.
- The % of REM sleep decreases slightly but steadily over the lifespan from about 25% age 5 to about 19% at age 70.
Stern and Morgane (1974) who suggested that REM sleep functions for the restoration of neurotransmitters in the brain. The high level of REM during prenatal and early post natal life serve the purpose of stimulating the brain to help it grow and become more organised (Breed-love et al (2007)). So maybe we learn less as we age. This might suggest we need to sleep less as we grow older.
Evaluation of lifespan changes in sleep
Research into lifespan changes in sleep has been conducted in numerous sleep labs. The changes in quality and quantity of sleep throughout life have been replicated and are well established.
Older people may not have a reduction in sleep as established by research as older people often cat nap during the day. Their total number of hours asleep may be similar to that of a younger adult.
The methods used in sleep study research may itself disrupt sleep. For example P's have to wear electrodes on the scalp while an EEG measures brain activity. The sleep that people have in a lab sleep study may not be a true representation of their sleeping patterns at home.
Borbley (1986) warns against the use of generalisations in sleep patterns for different age groups. He found that P's would show the same sleep patterns on different nights but that there was very little consistency in sleep patterns between people of exactly the same age.
The Evolutionary Theory of Sleep
Meddis (1975) suggests that sleep has evolved as an ADAPTIVE behaviour. Evolutionary theories suggest that sleep evolved to aid survival in the evolutionary past. Its function is to protect us from hard, prevent accidents and conserve energy at a time when itwould be inefficent for us to be awake because it is too dark to look for food or do any other beneficial activities.
Many animals and humans sleep during the time of darkness as we do not function effectively at this time. Also, remaining quiet and relatively still while asleep protects us from predators.
Meddis suggested the amount of time animals spend asleep depends on the safety of their sleeping habits and how much time they have to spend searching for food. Herbivores spend a lot of their day grazing for food.
Evaluation of evolutionary theory of sleep
A strength of ET is that it explains why different animals in the same category can have completely different sleep patterns. For example, lions sleep for large periods of the day and gazelles only sleep for a few hours because they are prey.
If the purpose of sleep is to keep us safe from predators, then it doesnt make sense that we should be in a semi-unconcious state as this would make us more vulnerable to predators. Also, it raises the question as to why creatures who are not preyed upon need so much more sleep.
Sleep deprivation studies seem to indicate that sleep serves a more complex function than simply keeping us safe from harm. Some animals will die if totally deprived of sleep which also shows that sleep has multiple purposes
If the only purpose of sleep is to keep us safe then does sleep consist of a complex pattern of stages consisting of REM and NREM sleep?
Restoration theory - Oswald (1980)
Oswald (1980) suggested that sleep serves as a period of physical rest during which we can recuperate and which promotes physiological processes that rejuvenate the body and brain. He proposed that sleep is a time during which we eliminate waste products, repair cells and restore depleted reserves of energy. Body replenishment occurs during NREM sleep and brain replenishment occurs during REM sleep.
Consistent with this theory is that growth hormone is released into the blood during NREM sleep. This hormone plays a role in the synthesis of proteins which is an important process in the restoration of body tissue. Oswald sees REM sleep as essential for brain repair and restoration.
The case study of Randy Gardner - He stayed awake for 264hrs in 1964. He developed blurred vision and incoherent speech, some disturbances, such as thinking innanimate objects were human, and a mild degree of paranoia. He slept for 15hrs on the first night. Recovery was specific to certain stages -2/3 of stage 4 SWS and hald of REM sleep were recovered.
Due to lack of control its difficult to determine whether microsleeps had occured. Lacks control of extraneous variables which are necessary to draw conclusions about the ill effects of sleep dep.
Hornes Core Sleep Optional Model
Hornes model suggests that body replenishment occurs during relaxed wakefulness, whereas Oswald suggested that occured during stage 4. Horne suggested that stage 4 and REM is for brain replenishment and is 'core sleep' where as light sleep is 'optional sleep'.
Animal studies - Studies such as Rechtstaffen provide evidence that sleep does serve an important restorative function. It seems most likely that sleep deprivation interferes with the immune system which then leads to death.
Rechstaffen put 2 rats on a turntable. The experimental rat had 13% sleep whereas the control rat had 69%. The experimental rats showed rapid decline in health and 21 out 33 died after 2 weeks. This study supports restoration theory as it shows that sleep is essential for health.
Stern and Morgane (1974) proposed that REM sleep is specifically for the restoration of brain neurotransmitters.
Research Study: Ozturk et al (1999)
Ozturk investigated if sleep deprivation adversly affects the immune system. The used two groups; a control group and a sleep deprived group. They were all given a baseline sleep. The following night the sleep deprived group were not allowed to sleep for 48hrs. At the end of this they had a recovery sleep.
- The results from this study showed, during recovered sleep the deprived P woke less frequently than normal. The amount of delta sleep increased, whilst the length of stage two decreased.
- The natural killer cells (NK) in the blood samples decreased by around 37% during the first 24hrs of sleep dep, however this did not decrease but remained the same during the next 24hrs of deprivation. The levels of NK cells returned to normal following the period of recovered sleep.
Strengths - There was a clear control group with which the sleep deprived could be compared
Weaknesses - This study is unethical as lack of sleep can cause stress on the body to the Ps.
Evaluation of restoration models of sleep
- The fact that young babies sleep much longer than adults do is also supportive of the theory.
- Studies of sleep dep provide additional support. When animals are repeatedly deprived of REM sleep they find it more difficult than usual to learn. Also, total sleep dep studies in rats resulted in their death after 30 days. It has also been established that REM deprivation in humans interferes with memory.
- There is physiological evidence to support this theory. Growth hormone is produced during NREM sleep in both children and adults and growth hormone is important in the restoration of body tissue.
- According to this theory, it would make sense that if we have a very physically innactive day we should need less sleep. Studies have not supported this.
- When we are deprived of sleep for some time, we do not need to make up for all the lost sleep. This was demonstrated in the case of Randy Gardner.