Somnolence or drowsiness was a strong or persistent desire for sleep, or sleeping for an unusually long period of time.
In 2259, Una Chin-Riley was given a gas sedative after suffering an intrusion on her abdominal aorta, and was informed by Nurse Christine Chapel that she would start to feel drowsy. (SNW: "Memento Mori")
In 2374, Seven of Nine brought The Doctor the results of astrometric scans, Captain Janeway having suggested they could aid in uncovering the cause of the somnolence that was affecting USS Voyager's crew. (VOY: "Waking Moments")