When and why did sleep evolve?

Sleep is familiar to all of us, as a state we enter into and exit from daily with seemingly boring regularity. Yet, despite its familiarity, many aspects of sleep remain mysterious. 

A recent centenary article in the Journal of Experimental Biology, authored by Krishna Melnattur, Assistant Professor of Biology and Psychology at Ashoka University, takes a comparative approach to the study of sleep. It examines sleep across various taxa to identify unifying themes. While our personal experience of sleep and dreams might lead us to think of sleep as being a particularly human experience, it might surprise many to learn that sleep is nearly universal among animals (Fig. 1). Sleep states have been defined in animals ranging from mammals to cnidarians, such as jellyfish and Hydra – creatures lacking proper brains. Crucially, sleep in these animals with small brains and nerve nets has much in common with our own. Prof. Melnattur’s group at Ashoka University employs the fly Drosophila to understand mechanisms of sleep control and function. Flies, like humans, sleep through the night. Caffeine keeps flies awake, while antihistamines make them drowsy. Fly sleep also seems to perform similar functions – sleep-depriving flies impaired learning and memory. Recent research suggests that Drosophila sleep might also have distinct stages,  similar to humans, including a “REM-like” state where the brain activity resembles the awake state. Thus, the presence of multiple sleep stages might be an evolutionarily ancient phenomenon. Indeed, the article also highlights studies that describe multiple sleep stages in several invertebrate species, including the octopus and cuttlefish.

One interesting, albeit somewhat neglected, aspect of sleep highlighted in the article is the plasticity of sleep, meaning that it is modifiable. This plasticity of sleep is seen in response to ecological niches that species inhabit and the individual experiences of animals throughout their lifetimes. Some particularly striking examples of sleep plasticity are highlighted in Fig. 2. Ashoka researchers investigate this phenomenon in the lab by examining how different sleep modulatory inputs, such as social enrichment (housing animals in groups), immune challenge, starvation etc. are integrated into the fly brain. 

The article concludes with a discussion of the conservation of sleep function across taxa. In particular, the need for sleep to consolidate memories has been observed in animals ranging from mammals with billions of neurons to nematode worms with ~300 neurons. 

At Ashoka, Prof. Melnattur’s group, in collaboration with Prof. Debayan Gupta and the Mphasis Maker Space,  is exploring the effect of sleep on spatial learning. They are jointly developing a spatial learning assay that trains flies to navigate to a ‘cool’ spot on a warm plate using distal visual cues as guidance. Using a similar assay, Dr. Melnattur had previously demonstrated that aged flies exhibited declines in spatial learning.  Interestingly, these learning defects in aged flies could be reversed by enhancing the sleep of aged flies. These results are particularly exciting as they suggest that sleep can not only consolidate memory in a healthy brain but also restore functioning in an impaired brain. Sleep, therefore, holds the potential to function as a therapeutic agent.  Prof. Melnattur’s research group is actively investigating the mechanisms by which sleep carries out this restorative function.

Reference Article: Comparative biology of sleep in diverse animals, Journal of Experimental Biology, July 2023 | Volume 226 | Issue 14

Authors:  Rhea Lakhiani, Sahana Shanavas, Krishna Melnattur

Rhea Lakhiani and Sahana Shanavas are the two former ASP students.

Edited by Dr Yukti Arora