A boreal and temperate shark found over continental shelves and slopes. At high latitudes with cold surface waters it ranges into the littoral and even the intertidal (one large individual was found trapped in a tide pool) as well as the surface; however in lower latitudes with temperate water it becomes a deep-water, epibenthic shark, never coming to the surface and ranging down to at least 2,000 m in the extreme southern end of its range (off southern California and Baja California) (Ebert 2003).
Archived depth data from recent tagging studies in the Northeast Pacific showed some sleeper sharks regularly ascended and descended at rates over 200 meters per hour, traveling below the photic zone during the day and approaching the surface at night (Hulbert et al. 2006).
Development is yolk-sac viviparity, but pregnant females have yet to be found, and for some reason are rare, as in the closely related Greenland shark (S. microcephalus). This is possibly due to segregation of pregnant females beyond the usual fisheries gear that capture these sharks or extremely low fecundity, with a small fraction of adult females pregnant at any one time. Although female sharks with ovaries containing over 300 large unfertilized eggs and many small undeveloped ova have been captured in trawls off Moss Landing and Trinidad, California (Gotshall and Jow 1965, Ebert et al. 1987). Size at birth is between 40–65 cm TL suggesting that fecundity may indeed be high for this species given the enormous size of the females. Neonates have been taken in midwater trawls indiciating that they occupy this habitat early in life before taking up a more demersal lifestyle (Ebert 2003).
Mature Pacific Sleeper Sharks have not been reported from the Bering Sea and Aleutian Islands or the Gulf of Alaska, but we know of observers who have seen very large Sleeper Sharks, but they are usually just discarded.
The species attains a maximum length of at least 440 cm with unconfirmed records of up to 700 cm total length (TL) or more (Ebert et al. 1987, Ebert 2003). Adult females are 370–430 cm TL and larger individuals estimated at 700 cm TL or more have been photographed in deep water (Ebert et al. 1987, Yano et al. 2007). Males are mature by at least 397 cm TL (Ebert et al. 1987).
These sharks feed on a wide variety of surface and bottom animals, including flatfish, Pacific salmon, rockfish, harbor seals, octopi, squid, crabs, tritons, and carrion. It is not known if seals and fast-swimming pelagic fish such as salmon are captured alive by these lumbering, sluggish sharks or are picked up as carrion. The small mouths and long heads and oral cavities of these sharks suggest that they are powerful suction feeders, but this has yet to be observed. Pacific sleeper sharks commonly are attracted to traps set at great depths for sablefish (Anoplopoma fimbria), and get trapped themselves or else eat catch and bait-can and escape (Ebert 2003, Ebert et al. 1987).