HS (hora somni)
Formal Definition
Abbreviation of the Latin phrase "hora somni" meaning "at the hour of sleep" or "at bedtime"; used in medication orders to specify that a medication should be administered at bedtime regardless of the patient's typical sleep time; commonly applied to statins (often dosed evening due to circadian cholesterol synthesis), certain antihistamines for nighttime sedation (hydroxyzine, diphenhydramine), mirtazapine, trazodone for insomnia dosing, and certain long-acting opioids where consistent bedtime dosing improves sleep continuity.
How It's Used on the Ward
"HS" or "qHS" or "at bedtime" — a medication order that wants the patient to take it at bedtime, regardless of shift schedules; commonly written on statin orders (because cholesterol synthesis peaks overnight) and on sleep-aid prescriptions; the patient will often be reminded by their home caregiver to take the HS med right before bed.
Example
""Hospitalist: 'Continue his home atorvastatin 40 mg PO HS. Continue his home mirtazapine 15 mg PO HS for insomnia and depression. Both are bedtime meds — make sure nursing gives them at 2200 consistently, with his snack, not with his morning meds.'""
Clinical Context
Why HS for statins: cholesterol synthesis is highest overnight (3 AM peak), and statin half-lives are short enough that evening dosing achieves greater reduction in LDL. Simvastatin specifically FDA-recommended for evening dosing. For long-acting statins (atorvastatin, rosuvastatin), HS no longer evidence-based and any time of day works. ISMP/Joint Commission "do not use" list: "hs" is included due to frequent confusion with "qhs" (every bedtime — interpreted by some as "every hour of sleep"). Use "at bedtime" or a specific clock time (e.g., 2200). Mirtazapine HS dosing leverages sedative side effects; trazodone HS uses similarly. For hospitalized patients: HS administration often timed with nursing 2200 med pass regardless of patient's actual sleep. Risk: HS meds given to patients who fall asleep mid-med-pass may be missed or refused on waking.