– Deana Chandler, San Diego

ANSWER: “To sleep, perchance to dream,” Shakespeare's Hamlet pondered in his famous “To be or not to be” soliloquy. He was contemplating what happens after people die, but sleep researchers get equally ponderous over why we need sleep and whether sleep and dreaming are universal across the animal kingdom.

Even single-celled organisms have a circadian rhythm – a daily pattern of activity and inactivity. Spiders do, too. Some spiders are active during the day and inactive at night, and others are active at night and quiescent during the day. However hibernation is not the same as sleep, and an inactive spider is not necessarily asleep.

Sleep is defined as a period of inactivity with reduced responsiveness to sensory information that is rapidly reversible (unlike hibernation or a coma). To be considered sleep, the rest period must also be homeostatically regulated – that is, disrupting the rest period creates an increased need for sleep.

No published studies have measured changes in spiders' behavior when their rest period is disrupted to determine whether it meets the strict definition of sleep. Anecdotes from tarantula owners that their pets are sometimes difficult to rouse suggest that they sleep. The notion that spiders sleep (or at least that invertebrates sleep) is also supported by studies on fruit flies.

Fruit flies have periods of inactivity in which they are unresponsive to small vibrations that would ordinarily make them respond. During the inactive state, gentle tapping on their container disturbs them, and if they are deprived of a night's rest, they compensate by resting more the next day.

Not only does their behavior fit the definition of sleep, it has other similarities with sleep in mammals. Young fruit flies need more sleep than older fruit flies, and sleep is more fragmented in older flies. There are even mutant flies that are insomniacs. In addition, caffeine, antihistamines and amphetamines have effects on sleep and waking in flies similar to their effects in mammals.

Researchers are not particularly interested in drowsy fruit flies, of course. They are using fruit flies as a tool to understand the genetic and biochemical mechanisms that control sleep in the hopes of finding ways to treat sleep disorders in people.

No one knows whether fruit flies and spiders dream of each other – or anything, for that matter. No reports of invertebrate REM sleep, the type associated with dreaming, have been made.

Sherry Seethaler is a UCSD science writer and educator. Send scientific questions to her at Quest, The San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or e-mail sseethaler@ucsd.edu. Please include your name, city of residence and phone number.