Somnolentia means an abnormal state of drowsiness or sleepiness, with reduced alertness and a tendency to fall asleep. A somnolent person may be difficult to keep awake and, if severe, may not become fully alert when aroused.

It is a clinical sign, not a diagnosis. Possible causes include sleep deprivation, sedating medicines or alcohol, infection, low blood sugar, low oxygen or high carbon dioxide, metabolic disturbances, head injury, stroke, or seizures. Sedation scales describe somnolence as drowsiness or sleep from which the person cannot be fully aroused. Pfenninger and Fowler's Procedures for Primary Care, Third Edition, p. 4514-4517.

Seek emergency care immediately if new or worsening somnolence occurs with difficulty waking the person, confusion, breathing problems, bluish lips, weakness on one side, seizures, severe headache, head injury, or suspected overdose.

Somnolentia, according to K. S. Narayan Reddy, is a condition of incomplete sleep resembling drunkenness in its effects. Some mental faculties are abnormally excited while others remain inactive.

Reference: The Essentials of Forensic Medicine and Toxicology, K. S. Narayan Reddy and O. P. Murty, chapter on forensic psychiatry, section “Somnolentia” (listed around p. 379 in the current edition).