Sleep: Ubiquitous Yet Obscure

From William Shakespeare's sonnet 27:

Weary with toil, I haste me to my bed,

The dear repose for limbs with travel tired;

But then begins a journey in my head

To work my mind, when body’s work expired….

In the next few lines, Shakespeare talks about not being able to sleep because he cannot stop thinking of a beautiful youth. Although these lines are more aptly interpreted in the context of sleeplessness, one cannot help but admire the scientific depth of the first four lines in isolation from the rest of the sonnet. REM (rapid eye movement) sleep, the state of sleep most widely associated with dreaming, requires the brain to be more metabolically active than it is while it is awake or while it is in the NREM (non-REM) sleep state.

We all sleep. Every being that has a nervous system sleeps. Even neurons in a Petri dish display periods of quiescence (neuronal ‘OFF’ states), associated with sleep. From nematodes, fruit flies and jellyfish to dolphins, whales and elephants; all organisms display a need to sleep, although the extent and manner in which they do so, varies considerably.

Dolphins, whales, otariid seals, ducks, frigate birds and some other aquatic mammals and birds show a phenomenon of sleep known as unihemispheric slow wave sleep (USWS), where one half of their brain shows electroencephalographic (EEG) characteristics of slow wave sleep (SWS - a feature of NREM sleep characterised by tall and fat waves), while the other half of their brain is awake. The eye opposite (aka contralateral) to the awake half, often remains open while the eye contralateral to the sleeping half remains closed. This is also many a times accompanied by limb movements on the side of the body that lies contralateral to the awake half of the brain.

Why do some animals show such sleep patterns?

The reasons are many. Dolphins need to keep swimming to be able to breathe occasionally, even while they sleep. Thus unihemispheric sleep allows them to do so. Dolphins and other cetaceans predominantly show unihemispheric sleep, and no conclusive evidence of classical REM sleep has been found in them till date.

Pods of dolphins swimming in circles also display selective unilateral eye opening where the eye directed towards and beyond the pod member opposite to them remains open with the contralateral hemisphere being awake.

On the other hand, aquatic mammals such as seals can show bihemispheric SWS as well as REM sleep while they are on land. However, in water they switch to USWS. Northern fur seals show a characteristic swimming posture while in USWS. These seals lay on their side in the water with one side submerged in the water, and the other above the water surface. The eye, sensory vibrissae and limbs on the side of their body that is submerged remain active while they sleep, allowing them to keep swimming and also be aware of any predators approaching from inside the water. What allows these animals to display such elegant hemispheric switching behaviour between water and land, is a question that haunts many researchers. Sea otters may also sleep unihemispherically as they can also swim while sleeping.

Migratory birds such as the great frigate bird also show USWS while they enter long migratory flights. Some migratory birds enjoy the luxury to stop on the way and rest on the water surface before resuming their journey. However, frigate birds cannot do this as their overall body physiology is not suited well for flying after getting wet. Thus, during their non-stop flight, the only way to rest for them, is to sleep with half a brain, keep one eye open to monitor the surroundings and glide at the mercy of the winds.

Mallard ducks, while sleeping one by the other in a row show USWS such that only the birds sitting at the ends of the row are in USWS, while the others are in bihemispheric sleep. Interestingly, these unihemispherically sleeping birds have the eye directed away from the group open, indicating a sentinel function.

This brief discussion on unihemispheric sleep tells us that it is not necessary that during sleep, the whole body and brain remain inactive or in a state of rest. In fact, studies on rats have uncovered another fascinating phenomenon known as ‘local sleep,’ wherein neurons in some regions of the rat’s brain show sleep-like activity although the animal is awake.

Here it would be interesting to also talk about another means by which animals such as ground squirrels, some hamsters, mice living in cold regions and in unfavorable habitats conserve energy and enter a state of hypothermia (low body temperature). This state is known as torpor. Hibernation, a term one might be more familiar with, is actually a type of long-duration torpor. Although, a sleeping animal and a torpid animal may look similar; torpor and sleep are not the same. In fact, studies have shown that torpid or hibernating animals need to periodically emerge from hypothermia to catch up on lost sleep. Electrophysiological characteristics of recovery sleep post sleep deprivation and post torpor also show some similarities.

Sleep is therefore a ubiquitous phenomenon, but still obscure. Humans spend one-third of their lives sleeping. Our understanding of sleep has advanced considerably in the last 70-100 years, and yet, we can’t completely answer the questions of what, why and how sleep occurs.

Clues from sleep studies in heterogenous organisms and environments serve as valuable tools in elucidating the evolutionary history of sleep. This is especially important since the occurrence of resting behaviours is somewhat uniform across species, but its phenotypes as a result of adaptation show great variation.

References:

• Rattenborg NC, van der Meij J, Beckers GJL and Lesku JA (2019) Local Aspects of Avian Non-REM and REM Sleep. Front. Neurosci. 13:567.https://doi.org/10.3389/fnins.2019.00567

• Rattenborg, N., Voirin, B., Cruz, S. et al. (2016) Evidence that birds sleep in mid-flight. Nat Commun 7, 12468. https://doi.org/10.1038/ncomms12468

• Vyazovskiy, V., Olcese, U., Hanlon, E. et al. (2011) Local sleep in awake rats. Nature 472, 443–447. https://doi.org/10.1038/nature10009

• Rattenborg, N. C., Amlaner, C. J., & Lima, S. L. (2000). Behavioral, neurophysiological and evolutionary perspectives on unihemispheric sleep. Neuroscience and biobehavioral reviews, 24(8), 817–842. https://doi.org/10.1016/s0149-7634(00)00039-7

• V. V. Vyazovskiy, S. Palchykova, P. Achermann, I. Tobler, T. Deboer, Different Effects of Sleep Deprivation and Torpor on EEG Slow-Wave Characteristics in Djungarian Hamsters, Cerebral Cortex, Volume 27, Issue 2, February 2017, Pages 950–961, https://doi.org/10.1093/cercor/bhx020