Sleep states

What the board expects you to know:

Introduction

All birds and mammals sleep and other creatures have a dormant period during the 24 hour cycle, suggesting that sleep must perform some vital purpose.  Some herbivores such as horses and giraffes can sleep whilst standing but must lie down for REM when muscle paralysis sets in, otherwise they’d fall over.  Birds tend to have a much shorter cycle of sleep, and according to Wikipedia do not lose muscle tone to the same extent as mammals when they enter REM.

However, in humans the amount of sleep needed by individuals does show considerable variation.  Meddis (1979) reported the case of a woman who only slept for one hour per night but showed no ill effects. This case however is unusual and it is estimated that in the UK with an average of 7.5 hours sleep per night, that most of us are in a state of mild sleep deprivation.  Sleep deprivation studies highlight the need for sleep to maintain normal levels of awareness and cognitive ability as well as psychological health.  Three or four nights without sleep can result in symptoms of mild paranoia and hallucinations.  Yet, even in the most extreme cases, such as Randy Gardner’s eleven nights without sleep, the effects are not long lasting.

The nature of sleep

It is possible that you will be faced with a short question on the nature of sleep.  If so use the material on the stages of sleep (see the biorhythms booklet) covering stages 1 to 4 and REM and the characteristics of each.  Later in this booklet we will also look at lifespan changes and the way in which sleep patterns alter with age.  If this wasn’t enough it might also be possible to include material on sleep disorders, again to be covered later in this booklet.

Theories of sleep

Why then do we sleep, and why do we spend almost one third of our lives in this state of reduced consciousness?  There are two main theories

Evolutionary theories:

Sleep helps to protect us from harm at night

Sleep helps us to conserve energy

Restoration:

Sleep helps us to repair damage done to our bodies during the day

Sleep restores the brain’s levels of neurotransmitters

Evolutionary (ecological) theory

‘The lion and the lamb shall lie down together but the lamb will not be very sleepy!’

Woody Allen (from Love and Death)

1. Protection (Meddis 1975)

In our evolutionary past night time would have been a time of great danger.  Since as a species we have poor night vision we would have been unable to forage, likely to fall and hurt ourselves and wide open to predation from species with better night sight.  Sleep would have been an evolutionary advantage since it would have kept us out of harms way.  As a result, those members of the species that slept would have been more likely to have survived to maturity and passed on their genes, ensuring that as an activity, sleep would have been retained in our behavioural repertoire.  The theory also considers the metabolic rates of other species, predicting that animals with high metabolic rates will need to spend more time eating so have less time to sleep.

Animals such as the shrew are safer since they have a burrow to return to, but due to their high metabolic rate (heart rate of 800 beats per minute) and need to be eating constantly only have time to sleep for two hours per day.  Generally speaking smaller species have higher metabolic rates because of their large surface area to volume ratio.  This results in loss of a lot of heat energy in comparison to species that are larger.

Larger preyed-upon species, e.g. ground squirrel, have burrows where they are safe, similar to the shrew, but since they are larger and have a lower metabolic rate, they need to eat less often and so can spend longer tucked away in their burrows asleep. 

However, there are some glaring anomalies.  On the face of it you would expect species most at risk to sleep longer (in order to get added protection) but often the opposite is the case.  Species most at risk such as herbivores sleep least (a few hours a day in brief naps), whilst species that are at little risk such as big cats sleep for most of the day!  Since this can’t be explained by one aspect of the theory (protection), food intake is used instead.  The lion feeds once every few days so spends the rest of its time asleep… because it can!  Herbivores with their impoverished diet of grass need to be eating all the time so don’t have the time to sleep.

Other obvious evaluation comments

If the only purpose of sleep is to protect from harm, then why do species that face the most        risk when asleep bother to sleep at all.  Surely it would make more sense to stay awake and alert to danger.  Research in India for example has suggested that given a choice, lions are happier tucking into a sleeping human than a more active one!  Evans (1984) sums it up nicely: ‘The behaviour patterns involved in sleep are glaringly, almost insanely, at odds with common sense.’ 

Sleep can also be dangerous in other respects as these two dolphin examples illustrate:

The Indus dolphin is at constant risk from being hit by logs and other big river debris being swept down the River Indus.  Clearly, loss of consciousness is life threatening since it means loss of vigilance.  However, despite this it still grabs quick naps of a few seconds at a time.  In effect, this dolphin is risking its life to sleep.  How can this be protective?

The Bottlenose dolphin sleeps with one hemisphere of its brain at a time (unihemispheric slow wave sleep) so it can remain partly conscious and return to the surface to breathe.  This takes place in two hourly cycles with one half of the brain always remaining fully conscious.  The fact that it has evolved such a bizarre sleep pattern suggests sleep is serving an essential purpose. 

According to the song

‘The lion sleeps tonight’

…in fact it sleeps for most of the day too!

If sleep is there to protect us from predation why would a creature at the top of the food chain with no natural predators spend up to 22 hours a day asleep?

Evaluation

Meddis’ theory does try to explain the diverse sleep patterns found across various species, unlike the restoration theory that we’ll look at in a while. 

Siegal (2005) in reviewing the data on different sleep patterns, varying from a few hours per day in sheep and herbivores to 20 hours or more a day in lions, bats and opossums believes it’s difficult to see how sleep can be serving the same purpose in all species.  Killer whales and dolphins barely sleep at all in the first few months of life.  Compare this to the human infant that sleeps 18 or more hours a day during the same stage of development.  How can sleep be serving the same purpose in both?  In humans it is assumed that we sleep more to aid in development.  If so, perhaps dolphins take care of these developmental issues whilst the young are still in the womb so don’t need the extended sleep pattern once born (Blumberg).  Horne also suggests that sleep may be serving a different purpose in different species.

If sleep is designed to make us inconspicuous at night, why do we snore! (Bentley 2000).

2.  Conservation of energy (Webb 

A variation on Meddis is the Hibernation Theory which is also sees sleep as an adaptive behaviour, but this time designed to conserve energy.  It compares sleep to hibernation.  During hibernation body temperature falls and the animal becomes inactive as a way of conserving energy when food is scarce.  The more at risk we are from predators the longer we will sleep.  Other factors will also effect the time spent sleeping, for example the time we need to spend each day searching for food.  Again, in the case of early human species night time would have been an unproductive period when we would have been unable to forage.  Sleep would have been one way of conserving our resources by lowering our metabolic rate. 

Evaluation of conservation theory

Research has shown that energy metabolism is significantly reduced during sleep (by as much as 10 percent in humans and even more in other species). For example, both body temperature and caloric demand decrease during sleep, as compared to wakefulness. Such evidence supports the proposition that one of the primary functions of sleep is to help organisms conserve their energy resources.

Meddis criticises the theory on the grounds that it is over-simplistic.  According to Meddis (as seen above), the amount of time spent sleeping is a compromise between protecting from danger and dietary requirements. 

Just being inactive at night would save almost as much energy but without the added danger of loss of vigilance.  It is estimated that the calories we save by sleeping rather than simply resting, is equal to the calories in a slice of bread, though it is higher in other species. 

Evaluation of evolutionary theories in general

On a positive note, the evolutionary theories do attempt to explain the sleep patterns of various species and generally they are able to predict the sleep times of species.  However, to do this they have adopted a catch-all approach.  For example with Meddis, if threat of predation doesn’t work then metabolic rate will! 

If sleep serves no other purpose other than safety, why do we suffer psychological problems when deprived of sleep and why as Rechtschaffen found in rats do animals die without sleep.

Empson (1993) describes sleep as ‘a complex function of the brain involving far reaching changes in body and brain physiology’ adding that it must have some restorative function.  He famously refers to the evolutionary theories as ‘waste of time’ theories as they see sleep merely as a way of passing time.

In an attempt to explain REM in evolutionary terms it has been suggested that active sleep is most prominent in birds and mammals - both warm-blooded.  Perhaps REM keeps brain active and prevents it dropping to dangerously low temperatures.

Evolutionary theories are unable to explain the complexities of sleep.  For example why do we have five stages of sleep (including the very bizarre REM stage)?

Finally, some have argued that sleep would now be pointless in most human societies because we are much more advanced and able to protect ourselves against harm at night.  However, as already pointed out, our change in behaviour as come about very quickly (in evolutionary terms), particularly with the discovery of electricity.  Evolution of biology and physiology on the other hand is much slower, so we wouldn’t expect to see big changes in our sleep pattern for hundreds of years at least: the genome lag

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