The yearly period of deep sleep, lasting about six days, that takes the place of nightly rest for Denobulans and from which Doctor Phlox proved extremely hard to wake.
Denobulans need very little ordinary sleep. Instead they hibernate once a year for roughly six days, and the cycle is not easily postponed or interrupted.
Enterprise shows the consequences in Two Days and Two Nights. While most of the crew take shore leave on Risa, Phlox settles down in sickbay for his annual sleep. Travis Mayweather is then brought back injured after a climbing accident and suffers an allergic reaction to the painkiller he was given at a Risan hospital, and the medical staff have no choice but to rouse the doctor two days into his rest.
The Phlox who answers is groggy, rambling and barely able to stand. With T'Pol and Ensign Cutler steering him he manages to work out a treatment before falling asleep again on the spot. The series also establishes other oddities of Denobulan biology, including a face that inflates like a puffer fish when the owner is threatened.