Elves need to meditate 4 hours a day to be in shape according to the PHB.
Trance: Elves do not need to sleep. Instead, they meditate deeply for 4 hours a day. (The Common word for such meditation is “trance.”) While meditating, you can dream after a fashion; such dreams are actually mental exercises that have become reflexive through years of practice. After resting in this way, you gain the same benefit that a human does from 8 hours of sleep.
But must those 4 hours/day be consecutive?
Some underlying questions are:
If elves have to meditate 4 hours consecutively to gain any benefit, this means that if someone wakes you up after 3 hours and 59 minutes, they have to start it all over?
If the trance is part of a long rest, do elves have to meditate at least 4 hours within a period of 8 hours to gain benefits and can those 4 hours be separated? (Whether the Long Rest is over or not after said 4 hours is not relevant here and was already answered: Is 4 hours long enough for a long rest for Elves and Warforged?)
If meditating is done only as the equivalent of sleeping and has nothing to do with Long Rests, could an elf sleep an hour in the morning, two after lunch and one final hour in the evening and stand watch all night? (Possibly skipping a Long Rest).
Am I forgeting something?