On planets with eternal day or night, how does alien life know when to sleep?

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On Earth, the human body knows when to sleep and when to wake up when day changes into night and night changes into day. But what if that doesn't happen? Scientists say there are billions of such planets in the universe where the day-night cycle does not occur. This is because they are tidally locked to their star, which means only one side faces the star while the other is always in the dark. 

Research published in The Conversation has now delved into the ways in which alien life, if any, lives, sleeps and evolves on such planets. 

Human beings, and many other life forms on Earth, go to sleep and wake up because of the circadian rhythm that is directly linked to day and night. But how does alien life know when to sleep or when to wake up on planets where there are no days or nights? Scientists say that the condition for a circadian cycle there likely depends on other factors, instead of day and night. 

A circadian clock based in space instead of time is one possibility when organisms living on a planet's dayside migrate to the nightside to rest and regenerate. 

The paper says that the circadian cycle affects biochemistry, body temperature, cell regeneration, behaviour and much more. However, it is not known how important periods of inactivity and regeneration are to life.

The researchers cited organisms on Earth that continue to evolve and grow without being linked to daylight, such as cave-dwellers, deep-sea life and microorganisms in the Earth's crust and the human body.

They have biorhythms but ones that are not linked to light. For eg, naked mole rats live underground and are never exposed to the sun. However, they do have circadian clocks but ones attuned to daily and seasonal cycles of temperature and rainfall. Deep-sea mussels and hot vent shrimp synchronise with the ocean tides.

The paper says that such planets probably also evolve biorhythms synchronised to such cycles. On planets tidally locked, the contrast between dayside and nightside seems to generate rapid jets of wind and atmospheric waves.

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When these elements interact with each other, the climate changes to different states, causing regular cycles in temperature, humidity and rainfall. So, even though the planet is fixed, the environment is changing.

Why do some planets have only day or night?

As per a study published in The Conversation, there are between 100 billion and 400 billion stars in the Milky Way. Most of them are cool red dwarfs, also known as M-dwarfs. As per a 2013 study, 41 per cent of M-dwarf stars have a planet orbiting in their "Goldilocks" zone, that is the distance at which the planet has the right temperature to support liquid water.

Rocky planets that orbit an M-dwarf's habitable zone are called M-Earths. Since M-dwarf stars are much cooler than our Sun, the planets are close-in, making the gravitational pull of the star on the planet immensely strong.

As the star's gravity pulls harder on the near side of the planet than the far side, the planet's spin slows down. This means most M-Earths are probably tidally locked, with one hemisphere always facing the sun while the other always faces away.

The same is the case with the Moon, which is tidally locked to Earth, which is why we never see the far side of our lunar satellite. The nearest such planet to Earth is Proxima Centauri b located in Alpha Centauri. 

So, M-Earths have no days, no nights and no seasons.