Daily briefing: Animals without brains sleep too — hinting at why we sleep at all

• Cnidaria, including jellyfish and sea anemones, exhibit sleep-like behavior despite lacking a centralized brain, possessing only simple neural networks along their bodies (ScienceAlert, Science Magazine).
• These animals show reduced activity and decreased arousal, similar to human sleep, for approximately one-third of the day (ScienceAlert, Science Magazine).
• When disturbed during sleep, jellyfish take longer to respond, analogous to a groggy human (Science Magazine).
• Sleep deprivation in jellyfish and sea anemones leads to an increase in neuronal DNA damage and prompts them to sleep longer the following day (ScienceAlert, Science Magazine).
• Both species show increased sleep duration when exposed to DNA-damaging agents like UV light or chemotherapy medication (ScienceAlert, Science Magazine).

• The presence of sleep in brainless organisms like jellyfish suggests that sleep is an extremely ancient biological process, predating the evolution of complex brains (ScienceAlert, Science Magazine).
• This research challenges the long-held belief that sleep primarily evolved to manage complex brains, instead supporting the idea that its origin lies in the need for early neurons to repair themselves (Science Magazine, Reddit Science).
• The findings suggest that a fundamental purpose of sleep, even in simple nerve nets, is to facilitate efficient cellular maintenance and repair DNA damage accumulated during wakefulness (ScienceAlert, Reddit Science).
• The evolution of sleep, despite its inherent risks like increased vulnerability to predation, must offer a powerful benefit to counterbalance these compromises (ScienceAlert).

Focus areas by different sources:

• ScienceAlert: Highlights that the balance between DNA damage and repair is insufficient during wakefulness, and sleep provides a consolidated period for cellular maintenance, driven by DNA damage and cellular stress in simple nerve nets.
• Science Magazine: Emphasizes the remarkable similarity of jellyfish and sea anemone sleep patterns to humans, including the effect of melatonin and increased sleep after deprivation. It quotes experts stating that sleep likely arose with the first neurons.
• Reddit Science: Directly states that jellyfish sleep like humans despite lacking brains, bolstering the theory that sleep evolved to protect DNA in individual nerve cells.