Sleep is a big part of human lives. A healthy night’s sleep is meant to be seven to nine hours per night, making up roughly one third of our 24-hour day. Whether or not the majority of the population (especially students) get this amount per night is highly unlikely. But, human society indulges the subject of sleep, with discussions about dreams or how tired a person is, being the norm. People can be overtired, can sleep ‘too much’ and can have their entire day changed by whether they slept well the night before.
It is therefore no real surprise that humans are obsessed with sleep – many important neurological and physical activities are carried out while humans’ slumber. Knowledge is consolidated, metabolic waste is eliminated from the brain, hormones are produced, energy stores are replenished. There’s a lot going on.
It has been known for a long time that humans experience two separate sleep states. There is slow-wave sleep and ‘rapid eye movement’ or REM sleep. Look at the collected information from most wristband-based fitness devices and they’ll give you an idea of your sleep stages and how long you spend in each. It can decipher the difference between the two based on heart rate and physical movement as REM is associated with dramatically reduced movement of the body, but increased heart rate compared to slow-wave sleep. REM is also the stage at which dreams occur.
Other land mammals, such as dogs, horses, cats and elephants, as well as birds, are also known to experience the same two sleep states as humans. It has always been believed that this was limited to these groups. Recently, however, a group of scientists have discovered analogous sleep states in a very different animal.
A team of researchers from the Centre national de la recherchescientifique (CNRS) and ClaudeBernard Lyon 1 University have been analysing the sleep states of the Argentine tegu (a type of lizard). They started by replicating a study in 2016 that watched the sleep states of thebearded dragon lizard. They confirmed that both lizard species have analogous sleep states to the slow-wave and REM periods of warm-blooded animals. Delving deeper, however, they found that the characteristics of the two species’ sleep states were not only very different from mammals and birds, but quite different from each other.
In humans, REM sleep is characterized by cerebral and eye activity similar to that occurring while awake. Hence, the vivid dreams and the increased heart rate associated with REM. In both lizard species, however, the equivalent sleep state had slower eye movements. In the tegu, the cerebral activity was also very different, not at all resembling that of waking hours. Although the precise neurological activity was not elucidated, the scientists assume that the differences in behaviour reflect differences in mental and physical activity.
Despite the observed differences between sleep behaviour, and the presumed physiological differencesthat these reflect, the presence oftwo sleep states does still suggest an evolutionary link. The similarities in sleep states between land mammals and birds has always presented a puzzle, with the best theory suggesting that they must have arisen independently in the respective ancestors of the two groups (this is an example of ‘convergent evolution’). However, the existence of equivalent states in lizards bridges the gap, suggesting that the states arose in a common ancestor of all three groups. This could run in the same thread as the discovery that scales, hair and feathers also all have a common ancestor.
Although the similarities between lizard sleep states and those in land mammals and birds suggest a common ancestor, there has clearly been quite a lot of changes betweenthe groups since then. Nonetheless,while scientists are still trying to fully understand human and mammalian sleep, the lizard sleep states provide some indication of just how long-ago sleep states developed.
Image credit: Bernard DuPont via Flickr