Researchers say that real sleep — measured by changes in electrical brain activity — occurs only in warm-blooded animals with well-developed forebrains. But you can find something that looks like sleep, which manifests itself as a sustained lack of movement, in creatures as ancient and lowly as the cockroach. Roaches have regular active and inactive periods; if you force a roach to increase its active time, it will respond by increasing its periods of inactivity, just as a higher animal will respond to a period of sleep deprivation by snoozing longer than usual.