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SLEEP AND HEALTH
2.5 ANIMAL STUDIES
As pet owners know, cats sleep (most of the time it seems) and so do dogs. But do
fish sleep? And flies? Yes, most animals sleep, and they even show the same phenomena
as in humans; from deep sleep, dream sleep to sleep disorders. There are
also many differences and weird behaviour, such as sleeping with only one half of
the brain at a time (dolphins and ducks).
As Ising points out (Appendix 3), in animal experiments it is possible to assess the
complete causal chain from noise exposure via physiological reactions and biological
risk factors to morbidity or even mortality. However, a quantitative application
of the results to humans is not possible. Instead, the method is useful in studying
the pathomechanisms qualitatively. Rechtschaffen and Bergmann (1965) studied
sleep deprivation in rats, showing that total sleep deprivation leads to mortality
in 16 to 20 days. As the animals in the last stage died from microbial infection,
Everson and Toth (2000) proceeded to show early infection of the lymph nodes
and other tissues and hypothesized that daily sleep of some amount is necessary to
maintain an intact immune system that will prevent bacterial invasion, a view that
has been challenged.
Surprisingly, sleep in the common fruit fly – Drosophila melanogaster – has many
similarities with mammalian sleep, including sleep deprivation leading to impaired
performance. Genetic studies in fruit flies (Cirelli et al., 2005) led to mutant flies
that can get by on 30% less sleep than their normal counterparts, thanks to a single
mutation in one gene. While they sleep 30% less they show no immediate ill
effects. The lifespan of the flies is, however, reduced by 30%.
These animal models certainly lead one to believe that sleep is a biological necessity,
and tampering with it is dangerous for survival.
As Ising shows (Appendix 3) noise may play a role in this. Under stressful circumstances
the death rate of rats is increased when noise levels are increased from
“ambient” to L eq =69 dB(A). Are noise and sleep deprivation stressors that both
lead to early death? Is the noise effect due to sleep deprivation? A carefully
planned study may sort this out. The question still remains, however, as to how far
this is relevant to humans.
2.6 CONCLUSIONS
From the evidence presented so far it can be deduced that sleep is important for
human functioning. Why exactly is less evident, but it is clear that disturbed sleep
(either from internal factors or from external factors) leads to or is at least associated
with fatigue, lower cognitive performance, depression, viral illness, accidents, diabetes,
obesity and cardiovascular diseases. Animal experiments show that sleep deprivation
shortens lifespan. The fact that – in comparison– relatively mild effects turn
up in human sleep deprivation experiments could be due to the short period (about
10 days in controlled experiments) and the limitation to young and healthy adults.
The central position of sleep in human functioning is summarized in Fig. 2.1. In this
figure relations with sufficient evidence are indicated with solid lines, while relations
for which limited evidence exists are indicated with interrupted lines. Feedback connections
are in red and double-dotted.