What if a day lasted 48 hours?

Every day the Sun comes up. Later it goes down, and it is night.

One day and one night take about 24 hours. What if they took 48 hours?

Morning, afternoon, evening, night: the pattern repeats every day of your life. It happens because Earth spins once on its axis about every 24 hours, turning each place toward the Sun and then away from it.

Now imagine Earth spun half as fast, so one day and night lasted 48 hours. This is a thought experiment: nobody can slow Earth down, but scientists know a lot about how living things keep time. Choose a path to explore what might change.

NASA gives Earth's rotation period as 23.9 hours and the length of a solar day, sunrise to sunrise, as 24.0 hours. Life on Earth has evolved under that rhythm, and many organisms carry internal clocks matched to it.

This thought experiment doubles the day. It is not a simulation: each step separates established findings about biological clocks (fact) from reasoned guesses about a world that has never existed (idea).

Your body has a clock inside. It helps you feel awake in the day.

It helps you feel sleepy at night. It is set for a 24-hour day.

Your body has its own clock, called a circadian rhythm. It makes you feel sleepy at night and alert in the day, and it follows a cycle of about 24 hours. Light and dark set it more than anything else.

In the evening, a part of your brain called the master clock tells your body to make more of a substance called melatonin, which makes you sleepy. Most living things have clocks like this, including animals, plants and even tiny microbes.

Circadian rhythms are physical, mental and behavioral changes that follow a roughly 24-hour cycle. In humans a master clock, the suprachiasmatic nucleus in the brain, coordinates clocks throughout the body and controls melatonin production according to the light reaching the eyes.

Harvard researchers measured the human clock's natural period at about 24 hours 11 minutes, varying by no more than about 16 minutes between people, far closer to 24 hours than the 25-hour estimates of earlier studies.

In a 48-hour day, daytime would be very long. Night would be very long too.

Your body clock would still want a 24-hour day. That might be confusing!

With a 48-hour day, a stretch of daylight could last about as long as a whole day and night does now, and so could the darkness. Your body clock, still set to about 24 hours, might make you sleepy in the middle of the bright afternoon.

People might sleep twice in each long day, or use lights and curtains to make their own 24-hour pattern. Plants and animals would face the same mismatch. Nobody knows exactly how life would cope, so these are ideas.

A doubled day would put the light-dark cycle twice as long as the internal period of most clocks. In a Harvard study, volunteers went to bed four hours later each day, creating a 28-hour day; this disconnected their biological pacemaker from clock time rather than resetting it.

That result suggests human clocks could not simply stretch to 48 hours. How people, crops and wild species would adapt over generations is unknown: these are ideas, not predictions.

Body clocks help living things keep time.

Sometimes a clock and the day do not match. Then people feel tired. Plants do not grow as well.

Body clocks follow a cycle of about 24 hours, set mainly by light. When the clock and the day don't match, problems follow. People flying across time zones get jet lag; shift workers' sleep is disturbed.

When body clocks fall out of step for a short time, people can feel drowsy and find it hard to concentrate. Plants are affected too: scientists found plants whose clocks matched the day grew better than plants whose clocks didn't.

NIGMS notes that jet lag and shift work alter the normal sleep-wake cycle, and that rhythms falling out of sync in the short term can bring drowsiness, poor coordination and difficulty with learning and focus.

In plants, Dodd and colleagues (Science, 2005) found that Arabidopsis plants whose clock period matched the light-dark cycle contained more chlorophyll, fixed more carbon, grew faster and survived better than plants with mismatched clocks. Matching the clock to the day is an advantage.

Other planets have days of different lengths. A day on Mars is a little longer than ours.

A day on Jupiter is less than 10 hours!

Days come in many lengths across the solar system. A day on Mars lasts about 24.7 hours, very close to ours. Jupiter spins so fast that its day lasts only about 10 hours. On our Moon, one day, from sunrise to sunrise, lasts about 709 hours, nearly a whole month.

A 48-hour day would be twice as long as the cycle that most body clocks on Earth follow.

NASA's planetary fact sheet lists the length of a solar day as 24.0 hours for Earth, 24.7 for Mars, 9.9 for Jupiter and 708.7 for the Moon.

The Martian day is close enough to Earth's to fall near the human clock's natural period, whereas a 48-hour day, like the Moon's, would be far outside it. A doubled day is a thought experiment; what it would do to life is a question scientists can reason about but not test.