Ranes
Adventurer
Actually, it was half that - 2 HP/level/day if you had nothing but bed rest, with the standard rate being 1 HP/level/day. On average, anyone would heal from beaten-nearly-to-death, up to full, in a number of days equal to their average Hit Points per level (so 2.5 + Con mod, for a wizard).
Yes and that's assuming a full day and night of bed rest, as you say, or the healing rate is half that again.