Heroes of the Realms
SHORT REST (1+ Hours): With a short rest (1+ hour), spend 1 or more HD, roll die, add Con modifier. After the roll, you can optionally spend more HD, up to your max HD. Regain HP equal to total. Once HD spent, long rest required to regain them.
LONG REST (8 Hours): Around 8 hours, no more than 2 hours on watch. Need at least 1 HP to take long rest. Only once per 24 hours. At end of rest, regain all hit points and half of your maximum number of Hit Dice.
At 1st level, if you take a long rest, you regain 1 HD back (despite rounding down of .5 = 0).
Not in rules but clarified by the designers:
Elves cannot use their Trance to get the benefit of long rest in 4 hours instead of 8; they still need 8 hours rest (they simply spend only 4 of it zonked out.).