Sleep is very ancient from an evolutionary perspective. The most primitive organism that is confirmed to have sleep-like activity (lethargus) is the nematode c. elegans. Some scientists believe that even simpler organisms like the box jellyfish have sleep-like states, but this is currently contested.
Caenorhabditis elegans /ˌseɪnɵræbˈdɪtɪs ˈɛlɛɡænz/ is a free-living, transparent nematode (roundworm), about 1 mm in length, that lives in temperate soil environments. The name is a blend of Greek (caeno- = new, recent + rhabditis = rod-like) and Latin (elegans = elegant). It was initially named Rhabditis elegans by Maupas (1900). It was then placed in the subgenus Caenorhabditis by Osche (1952) and then raised to generic status by Dougherty (1955). Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model organism
The hermaphrodite, which is actually considered a specialized form of self-fertile female because the soma is female but its germ line produces male gametes first, conducts the laying of eggs. Eggs are deposited externally through the uterus following internal fertilization. After hatching, juveniles pass through four stages (L1–L4). When crowded or in the absence of food, C. elegans can enter an alternative third larval stage called the dauer state. Dauer larvae are stress-resistant and do not age. Hermaphrodites produce all their sperm in the L4 stage (150 sperm per gonadal arm) and then switch over to producing oocytes. The sperm are stored in the same area of the gonad as the oocytes until the first oocyte pushes the sperm into the spermatheca (a chamber where the oocytes become fertilized by the sperm). The male can inseminate the hermaphrodite, which will use male sperm preferentially (both types of sperm are stored in the spermatheca). When self-inseminated, the wild-type worm will lay approximately 300 eggs. When inseminated by a male, the number of progeny can exceed 1,000. At 20 °C, the laboratory strain of C. elegans has an average life span of approximately two–three weeks and a generation time of approximately four days.
C. elegans is notable in animal sleep studies as the most primitive organism in which sleep-like states have been observed. In C. elegans, a lethargus phase occurs in short periods preceding each moult.