sleep as a phenomenon appears to have very old evolutionary roots. unicellular organisms do not necessarily "sleep", although many of them have pronounced circadian rhythms. the jellyfish cassiopea is the most primitive organism in which sleep-like states have been observed. the nematode "c. elegans" is another primitive organism that appears to require sleep. here, a "lethargus" phase occurs in short periods preceding each moult, a fact which may indicate that sleep primitively is connected to developmental processes. raizen "et al."'s results furthermore suggest that sleep is necessary for changes in the neural system.