Sleepers are distributed in tropical and subtropical waters throughout the world. They are so called because of their habit of resting on the bottom as though "sleeping", rarely moving unless disturbed. If not resting on the bottom, they often remain suspended and motionless in the water, diving down to hide when frightened or in danger.

Sleepers are closely related to gobies, although they lack the sucking disk that is customary in gobies and instead have separated ventral and pelvic fins. Most sleepers are fairly small, although the larger species have some food value. Sleepers are predatory in their feeding, hiding in weeds and crevices
in wait for fish.

The fat sleeper (Dormitator maculatus)can reach 2 feet in length but is usually less than a foot long. It inhabits brackish waters and freshwaters through the Caribbean and the warm Atlantic northward to the Carolinas.

Usually dark brown and mottled, it has a bluntly rounded head, a large mouth, no visible lateral line, and a rounded caudal fin. It bears a resemblance to a fat mullet, but its second dorsal and anal fins are large and of equal size.

The bigmouth sleeper (Gobiomorus dormitor) occurs along the Florida coasts, in the Caribbean, and also in freshwater. It can exceed 2 feet in length and is much thinner than the fat sleeper. It has a large, pikelike mouth and obliquely squared-off second dorsal and anal fins. The bigmouth sleeper has an olive-green body, and its first dorsal fin is outlined in black.

A 4-inch species, the blue sleeper (Isoglossus calliurus), inhabits the deep waters of the Gulf of Mexico; a 6-inch species, the emerald sleeper (Erotelis smaragdus), lives off the southern coasts of Florida and in the Caribbean, where
it blends with bright green algae.