London: Plants may need to sleep at night, a new study on primitive jellyfish suggests.

Researchers have discovered the first evidence of sleep in a species without a brain, leading scientists to suspect the habit is fundamental to life itself, not just animals.

An upside-down Cassiopea jellyfish photographed in Lake Macquarie. Credit:Steve Jones

The investigation focused on a jellyfish that spends its entire life sitting upside down on the ocean floor, doing nothing more than occasionally pulsing.

A team from the California Institute of Technology monitored the Cassiopea species closely, finding it went through periods of relative inactivity during the night, pulsing only about 39 times a minute compared with 58 times during the day.