The purpose of sleep is unknown. What is known is that it not only occurs in humans, but all mammals, and even as far back as box jelly fish and species such as the fruit fly Drosophila melanogaster. This suggests that sleep-like states are evolutionarily ancient. Now evidence suggests that a quiescent behavioral state exists in the nematode Caenorhabditis elegans, a round worm. This quiescent state develops during a developmental stage called lethargus, which temporally occurs before each of its four moults in larval stage transitions. 

Quiescence in C. elegans demonstrates certain specific characteristics of sleep. A key feature of sleep is reduced sensory responsiveness. This is manifested in C. elegans by decreased responses to abrupt mechanical and strident olfactory (smell) stimuli. This quiescent state is reversible, as is sleep. It also manifests a "homeostatic," or metabolically refreshing, quality when, after a period of enforced wakefulness, subsequent sleep occurs more rapidly and is deeper.

The temporal relationship between lethargus and the moult, which is essential for growth and development of the animal, is a period of enhanced biosynthetic activity. This is consistent with the sleep-like state playing a vital role in growth and development of the organism. Also related to lethargus are vital alterations in the nervous system and the connectivity between individual nerve cells. Connections between nerve cells are called synapses. The process of modification of these connections is called called plasticity. Plasticity is the basis for learning and memory and reflects what happens during the developmental stages in round worms. It is also a vital brain process in humans.

Strengthening of new neuronal connections is called consolidation and is necessary for the solidification of learning. This process requires expenditure of substantial amounts of cellular energy and must provide significant benefits. These synaptic modifications (plasticity and consolidation) associated with lethargus in C. elegans promote nervous system changes that are important in the organism's life cycle.  Such a role in nervous system development, maturation and function is notable in light of information suggesting that sleep, and sleep-like states, are necessary for the production of changes in the nervous system.

These findings highlight potential roles played by sleep in humans. It is well-known that sleep deprivation acutely diminishes mental functions. Chronic sleep deprivation is associated with elevations in the stress hormone cortisol. Cortisol, in excess, can cause shrinkage and loss of nerve cell connections (synapses). This occurs in association with aging and is more severe in dementing conditions such as Alzheimer disease. These findings are evidence of the importance of adequate sleep not only for provision of cognition, but also for the prevention of neuronal changes seen in severe memory loss states.