The person who needs an early night is still allowed to care about the gathering.

Leaving early gets read as a verdict on the evening. Whoever slips out at nine is assumed to have been bored, or to be less committed than the ones still there at one, though what they mostly have is a body with a different schedule. Staying late is a capacity, not a measure of affection.

If you are hosting, say near the start that people should leave when they need to. If you are the one going, say a proper goodbye and skip the excuse. Turning up for two good hours is a real contribution to someone’s evening, whatever the people ordering another round conclude.