Mammals, who are the main stars of this list, sleep to restore their mental and physical energy. The amount that they need depends on several factors, including age, body size, environment, diet, and the safety of their resting spot. Whether a mammal lives on land or in the sea can also affect how much sleep it needs.

Moreover, different mammals also spend varying amounts of time in non-REM and REM sleep. However, all of them studied thus far do exhibit signs of REM sleep, suggesting that mammals dream just like humans do!

Mammalian sleep is often categorized as monophasic or polyphasic.

The former describes animals who generally receive their sleep in one concentrated period of time. Humans are an example of monophasic sleepers — our circadian rhythms encourage us to sleep for extended periods at night and be active and alert during the day.

Polyphasic sleepers, on the other hand, tend to sleep multiple times throughout a 24-hour cycle. In nature, polyphasic sleep is more common, as many animals need to maintain some level of vigilance against predators. However, if threats are minimized, they can enjoy monophasic sleep. Marmosets, for example, sleep in trees surrounded by their family, enabling them to feel more protected and enter monophasic sleep phases.

Interestingly, birds sleep unihemispherically, with one side of the brain dozing off while the other stays awake. As they sleep, only the eye associated with the sleeping hemisphere of their brain is closed. Unihemispheric sleep allows them to protect themselves from predators. For example, mallard ducks can sleep in a row, and those at the end will be most likely to sleep unihemispherically, with their outward eye remaining open, while ducks in the middle sleep with both eyes closed.

Unihemispheric sleep also enables migratory birds to make their long flights. These animals may sleep while gliding, when their wings don’t need to flap as much. Birds, like the Alpine swift, have been documented flying for up to 200 days non-stop 

But these wholesome pictures might be beneficial to us even more than we realize. A study conducted by the University of Leeds in the United Kingdom, in partnership with Western Australia Tourism, has found evidence to suggest that watching cute animals may contribute to a reduction in stress and anxiety.

So thank you, 'Animals Being Sleepy,' for sharing them with us!