According to the PHB, concentration is broken under one of three conditions:
So my question is, assuming a spell that can be concentrated on for longer than 8 hours, can you concentrate through a long rest?
That depends on whether sleeping gives you the Unconscious condition.
According to the DMG, page 248, sleeping does give you the unconscious condition:
You can also apply conditions on the fly. They're meant to be intuitive for you to do so. For example, if a character is in a state, such as sleep, that lacks consciousness, you can say that the character is unconscious.
Crawford also says that sleeping gives unconsciousness.
So, you probably can't maintain concentration through a long rest, unless you're an elf! (Because they don't sleep.)
The currently accepted answer to this question opens with "That depends on whether sleeping gives you the Unconscious condition", which is correct. However, that answer was written in 2015. More than two years later Xanathar's Guide to Everything was published, and that reference book added optional rules for sleep that impinge on this answer, such that the answer to this question now crucially depends on whether these optional rules are in use.
XGtE says:
While a creature sleeps, it is subjected to the unconscious condition.
Here, a sleeping character is unequivocally unconscious. And, per the PHB:
An unconscious creature is incapacitated (see the condition), can’t move or speak, and is unaware of its surroundings
And per the PHB rules on Concentration:
You lose concentration on a spell if you are incapacitated or if you die.
Thus, if the optional rules from XGtE are in play, sleeping results in the unconscious condition, which means you are incapacitated, which means you lose Concentration.
If the DM is not using XGtE, it is their call whether or not sleeping ends Concentration, because in this case sleeping does not equal the unconscious condition. Rather, sleeping is a state to which a DM may assign consequences at their discretion.
From the DMG "Using and Tracking Conditions" (p. 248 - emphases mine)
Various rules and features in the game are clear about when they apply a condition to a creature. You can also apply conditions on the fly. They're meant to be intuitive for you to do so. For example, if a character is in a state such as sleep, that lacks consciousness, you can say the character is unconscious. Or did a character just stumble onto the ground? He or she is now prone.
This passage does not say that sleeping characters per force have the unconscious condition. Rather, the point is that a DM can choose to apply the mechanical effects of conditions 'on the fly', that is, when they feel the situation as such warrants it. In the case of sleep, it is not that a character is unconscious, but rather that they lack consciousness and thus can be treated for some situations as if they were unconscious (and note that this is a different interpretation of the DMG quote than given in the accepted answer).
One of the consequences of the unconscious condition is that a creature 'is unaware of its surroundings' and thus cannot be woken up by sounds, lights, or perhaps shaking. This is pretty unreasonable in the case of normal sleep, so most DMs would likely not apply this aspect of unconsciousness to a sleeping creature, but they would be within RAW to do so (and note that even though XGtE says that sleeping creatures have the unconscious condition, it also includes rules for them waking up in response to external stimuli).
Another consequence of unconsciousness is that attack rolls against such a creature have advantage. This seems like a reasonable effect to apply to a sleeping creature, but it would again be up to the DM to decide whether or not this applies.
Whether or not sleeping means that you are incapacitated enough to end Concentration on spells is a third potential consequence of unconsciousness and is thus similarly left up to the DM to decide if they are working only with the official PHB / DMG rules.
From the 5e Wiki:
Long Rest
A long rest is a period of extended downtime, about 8 hours long, during which you sleep or perform light activity: reading, talking, eating, or standing watch for no more than 2 hours of the rest period. If you take any strenuous action during a long rest, such as attacking, taking damage, or casting a spell, you must start the rest over.
Based on the fact that you're either sleeping or performing non strenuous actions, and that you cant cast spells during the long rest. I'd say that by RAW, no, you can't.
In the information for Long rest it says you can only read, talk, eat, or stand watch for 2 hours of the rest period, so for 2 hours of the rest you should be able to maintain concentration, but after that you'd have to sleep, which ends concentration.
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.
This means that elves require half the time that humans require for daily rest, but it also means that while sleeping you're in a trance. When defined, Trance means this:
Trance (noun): A half-conscious state characterized by an absence of response to external stimuli.
Meaning that during the time the elf is Trancing, they are semi-conscious, and based on the fact that you can't concentrate while unconscious, elves are partially unconscious.
I am in agreement with Lohoris about this however, this is some serious rules lawyering, and I'd houserule that for every hour that someone wants to concentrate that they have to roll concentration to maintain a spell effect ( DC20 + 2 per the number of hours they've been concentrating )
To prove this, we must prove both of these statements:
- Casting another spell that requires concentration. You lose concentration on a spell if you cast another spell that requires concentration. You can’t concentrate on two spells at once.
- Taking damage. Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher. If you take damage from multiple sources, such as an arrow and a dragon’s breath, you make a separate saving throw for each source of damage.
- Being Incapacitated or killed. You lose concentration on a spell if you are Incapacitated or if you die.
If you're an Elf, you do not become Incapacitated or Unconscious while in a Trance, and therefore do not invalidate the conditions for maintaining Concentration.
If you don't have a feature like Trance, then you cannot Concentrate through a Long Rest, as you'd be unconscious.
Note that the DM might still make you roll to maintain Concentration while in a Trance if he sees fit to decide it's hard to Concentrate and Meditate at the same time:
The GM might also decide that certain environmental phenomena, such as a wave crashing over you while you’re on a storm--tossed ship, require you to succeed on a DC 10 Constitution saving throw to maintain concentration on a spell.
A Long Rest is ended by doing something considered light activity for more than 2 hours, or strenuous activity for more than 1 hour.
A long rest is a period of extended downtime, at least 8 hours long, during which a character sleeps or performs light activity: reading, talking, eating, or standing watch for no more than 2 hours. If the rest is interrupted by a period of strenuous activity—at least 1 hour of walking, fighting, casting Spells, or similar adventuring activity — the characters must begin the rest again to gain any benefit from it.
Casting a spell for more than 1 hour is considered a Strenuous Activity. Note that casting a spell for longer than 1 action also uses Concentration. So there's a strong case for Concentration being at least Strenuous enough to invalidate a long rest.
At the very least Concentration would fall into a similar level of focus as keeping watch or reading. But you cannot keep watch for longer than 2 hours without losing the benefits of your Long Rest. (Yes even as an elf.) Note the phrasing used here:
Standing watch is even possible during it, but for no more than 2 hours; maintaining heightened vigilance any longer than that isn’t restful.
Concentration can be seen as a form of heightened vigilance.
Comparing Concentration to a light activity like reading, remember that you cannot stop focusing on the spell for even an instant, otherwise you lose it. While reading you can set down a book for a moment, or let your thoughts wander, and the book doesn't vanish: you can keep reading after.
This strongly implies that Concentrating on a spell for more than 1 or 2 hours would invalidate your Long Rest.
Therefore, even if you can Concentrate while taking a non-unconscious Long Rest, you wouldn't be able to gain the benefits of that Long Rest if you chose to Concentrate for longer than 1 or 2 hours, depending on how strenuous Concentration is in practice. Considering you can lose it, even from being hit by a wave on a ship, I'd lean towards 1 hour myself.
The Golden Rule overrides everything, and, frankly, if I were the GM I'd just tell you that obviously not: if you have to keep concentration, then you are not resting, period.
I'd also add that keeping concentration for so long might be difficult, so you'd have to make a few rolls to ensure you would actually manage it.
Concentration spells are not meant to be used that way, and this is clearly a situation where the GM must make some sense out of it, using his judgement.
