The evolutionary benefit of sleep has long been investigated. A lack of consciousness is not conducive to survival when a
predator comes around. So there must be some reason it has evolved as a critical part of life. But as of yet, no hypothesis
on its purpose, including sleeping as conservation of energy or sleeping to promote brain development and repair, has been
definitively proved in scientific studies.
However, a recent study by researchers from Germany, the United Kingdom, and the United States may have identified a verifiable
benefit of sleep. The researchers hypothesized that sleep has a great effect on immune function and specifically investigated
whether more sleep would result in less parasitic infestation, both on the micro level involving viruses, bacteria, and fungi
and on the macro level involving helminths, protozoa, and arthropods.
To test their hypothesis, the researchers looked at three variables. First, they tried to determine whether sleep has any
effect on the immune system by looking at whether animals with evolutionary-developed longer sleep periods had higher white
blood cell counts.
To determine the sleep durations of animals, the researchers picked 26 mammals along an evolutionary spectrum and extensively
researched the literature to find the average hours spent each day sleeping. The mammal with the shortest sleep duration in
the study was the horse at 2.9 hours a day. Hedgehogs had the longest sleep duration at 17.6 hours a day. To determine the
average white blood cell count of the 26 mammals, the researchers used the International Species Inventory System, which contains
reference values for various species.
The controls were red blood cells and platelets. If these two types of blood cells in each of the various species stayed at
about the same level, regardless of the average hours of sleep, it would show a positive correlation between more sleep and
higher white blood cell counts. And, indeed, white blood cell counts were significantly higher in those animals that slept
longer on average, while red blood cells and platelets remained unaffected. The researchers found that a 14-hour increase
in the amount of time spent sleeping resulted in 30 million more white blood cells per milliliter of blood (a 615% increase).
The specific immune cell types that increased in number were neutrophils, lymphocytes, eosinophils, and basophils. Monocytes
were the only cell type that was nonreactive to sleep duration.
Next, the researchers examined REM vs. non-REM sleep. They hypothesized that only non-REM sleep would be associated with increased
levels of white blood cells because much more energy is required for REM sleep. To determine how long the mammals they’d chosen
for the study remained in both REM and non-REM sleep, they searched the literature for data collected by electroencephalography.
Surprisingly, this aspect of their hypothesis was not supported. They found that increases in both REM and non-REM sleep resulted
in higher white blood cell counts. The type of sleep did not seem to matter.
Finally, since the researchers had established that increased sleep resulted in more white blood cells, they looked at whether
this truly translated into fewer parasitic infestations. They used data from the Global Parasite Database to find the prevalence
of various parasitic infections in 12 of the mammals they selected for the study. Their findings? Across the 10-hour range
of sleep duration within the dataset the researchers found a 24-fold decline in parasitism; species that slept for longer
periods were infected by fewer parasites. Thus, the researchers concluded that species that have evolved longer periods of
sleep have enhanced immune systems and more protection from parasitism.
This finding is in contrast to the idea that sleep is for energy conservation. Instead it is energy reallocation; the energy
is redirected during sleep to augment the immune system. The finding is also consistent with the fact that, when sick, animals
tend to sleep more, thus fueling their immune systems.
The researchers do not feel that sleep is solely for the benefit of the immune system. They think other benefits will be proved
in further research. But this study does provide a good bottom line for all of us: Get some sleep!
Source: Preston BT, Capellini I, McNamara P, et al. Parasite resistance and the adaptive significance of sleep. BMC Evol Biol 2009;9(1):7. [Epub ahead of print]