Science Magazine resurfaced a 2021 feature about an old but surprisingly difficult question: How do you recognize sleep in an animal that has no brain to scan?
How researchers test for sleep without a brain
Rather than looking for the electrical patterns used to classify human sleep, scientists look for a recurring period of reduced activity, weaker responses to the environment and a rebound after an animal is kept from resting.
In a 2017 experiment, upside-down jellyfish called Cassiopea slowed from about 60 bell pulses per minute during the day to 39 at night. The nighttime animals were slower to react when researchers lowered the false bottom beneath them. After currents of water disturbed them overnight, they were also less active the following day. Melatonin slowed their pulsing to nighttime speeds.
That combination resembles sleep by behavioral standards, even though these jellyfish have a ring of nerve-cell clusters rather than a centralized brain.
Researchers have reported similar clues in Hydra vulgaris, a tiny freshwater relative of jellyfish. Hydra were less active in darkness, and a later sleep-deprivation experiment found changes in the activity of more than 200 genes. Several of those genes are also involved in sleep in fruit flies.
“Sleep” remains a contested label
Not every researcher accepts that these states should be called sleep. Some prefer “sleep-like” because a still, unresponsive jellyfish or hydra cannot be compared directly with the well-defined stages measured in a human brain.
The disagreement is more than semantics. If animals without brains need sleep-like rest, memory consolidation and other brain-specific benefits cannot fully explain why sleep evolved. The research reviewed by Science points toward more basic roles involving metabolism, cellular maintenance and signals from tissues such as muscle and the gut.
Those ideas remain under investigation. Experiments in worms, fruit flies and mice have linked sleep loss or sleep regulation to energy balance, gut stress and muscle-produced proteins, but they do not establish one universal purpose for sleep.
The simplest cases are still unresolved
Placozoans and sponges would push the question further because they lack neurons altogether. Researchers have observed nightly slowdowns and cycles of contraction that might qualify as rest, but the 2021 feature described that work as preliminary or incomplete. Attempts to test puffball sponges were disrupted by the animals’ demanding care needs and the COVID-19 shutdown.
The jellyfish and hydra experiments suggest that a sleep-like state may not require a centralized brain. Whether the simplest animals truly sleep still depends on what scientists decide sleep must include.
