Do aliens sleep? You may take sleep for granted, but research shows that most habitable planets don’t have a day-night cycle. It’s hard to imagine, but there are organisms living in Earth’s dark habitats, deep underground or at the bottom of the sea, that give us a glimpse of what alien life without a circadian rhythm might be like.

There are billions of habitable planets in our galaxy. How did we arrive at this number? There are between 100 and 400 billion stars in the Milky Way. Seventy percent of them are tiny, cool red dwarfs, also known as M dwarfs. A detailed study of exoplanets published in 2013 found that 41% of M dwarf stars have planets orbiting in the Goldilocks zone, the distance at which a planet has the right temperature to support liquid water.

But these planets only have the potential to host liquid water. We don’t yet know if any of them actually have water, let alone life. In total, that’s 28.7 billion planets in the Goldilocks zones of M dwarfs alone. That’s not even counting other types of stars, like our yellow Sun.

Rocky planets orbiting in the habitable zone of an M-dwarf are called M-Earths. M-Earths are fundamentally different from our Earth. First, M-dwarf stars are much cooler than the Sun and are therefore close together, making the star’s gravitational pull on the planet extremely strong.

The star’s gravity pulls the near side of the planet more strongly than the far side, resisting this friction and slowing the planet’s rotation over eons until the rotation and orbit become synchronized. This means that most M-Earths are probably tidally locked, with one hemisphere always facing the sun and the other always facing away from the sun.

The year of a planet in tidally closed longitude is equal to its day. Since the Moon is tidally adjacent to the Earth, we see only one side of the Moon, not the dark side.

A tidally locked planet may seem exotic, but most potentially habitable planets probably are. Our nearest planetary neighbor, Proxima Centauri b (located in the Alpha Centauri system, four light-years away), is likely a tidally locked M-Earth.

So unlike our Earth, M-Worlds do not have days, nights, or seasons. However, life on Earth, from bacteria to humans, has circadian rhythms that are geared to the day-night cycle.

Sleep is just the most obvious. The circadian cycle affects biochemistry, body temperature, cell turnover, behavior, and more. For example, people who get their morning shots produce more antibodies than those who get them in the afternoon because the immune system’s response changes throughout the day.

We don’t know for sure how important periods of rest and regeneration are to life. Perhaps creatures that evolved without cyclical time could continue on their path without ever needing to rest.

To inform our speculations, we can look at organisms on Earth that live far from daylight, such as cave dwellers and deep-sea life, and microorganisms in dark environments, such as the Earth’s crust and the human body.

Many of these life forms have biorhythms that are synchronized with stimuli other than light. Naked blind individuals spend their entire lives underground, never seeing the sun, but have circadian clocks that are tuned to daily and seasonal cycles of temperature and precipitation. Deep-sea mussels and warm-mouth shrimp synchronize with ocean tides.

Bacteria living in the human gut synchronize with melatonin fluctuations in the host’s body. Melatonin is a hormone your body produces in response to darkness. Temperature changes caused by thermal vents, humidity fluctuations, and changes in environmental chemistry or currents can cause biological fluctuations in organisms. This suggests that biorhythms have intrinsic benefits.

Recent research suggests that M-Earths may have cycles that replace days and seasons. To examine such questions, scientists have adapted climate models to simulate what the environment on M-Earths, including our neighbor Proxima Centauri b.

In these simulations, the contrast between the day and night sides appears to produce fast wind jets and atmospheric waves similar to those that cause Earth’s jet stream to bend and twist. If the planet has water, thick clouds filled with lightning are likely to form during the day.

Interactions between winds, atmospheric waves, and clouds can cause regular cycles in temperature, humidity, and precipitation, changing the climate between different states. The duration of these cycles will depend on the planet, from tens to hundreds of Earth days, but they will not be related to its rotation period. The environment will change as long as the star remains in the sky of these planets.

Perhaps life on M-Worlds will develop biorhythms that synchronize with these cycles. If the circadian clock regulates internal biochemical oscillations, perhaps it should.

Or maybe evolution would come up with a more surprising solution. We can imagine species living on the day side of the planet and migrating to the night side to rest and recover. A circadian clock in space instead of time. This thought should remind us that if life exists, it will overturn assumptions we had no idea existed. The only certainty is that it will surprise us.