D&D Basic Rules v0.1 says on page 67 that a long rest is interrupted by "at least 1 hour of walking, fighting, casting spells". So does that mean that 59 minutes of combat doesn't interrupt the long rest? Or does any combat or spellcasting count as enough interruption to restart the rest?
A long rest is interrupted by giving up on it
The text is ambiguous, but 1 hour is meant to apply to the whole period of strenuous activity, while the list is there to define what types of activities could make up that strenuous activity period. The intention is that the rest has to be started over not just for any interruption, but for those unusual times when the party simply gives up on resting.
Mike Mearls clarified this on twitter:
The Seven-Sided Die @sevensideddie 9:30 AM - 9 July 2014
.@mikemearls How are Long Rest interruptions meant to work? The land is in confusion & dismay! http://forum.rpg.net/showthread.php… https://rpg.stackexchange.com/questions/42123…
Mike Mearls @mikemearls 10:01 AM - 10 July 2014
@sevensideddie interruption needs to be a full hour. Testers: "We rest 7 hours, a kobold knocks on the door, and now we have to start over?
The Seven-Sided Die @sevensideddie 10:04 AM - 10 July 2014
.@mikemearls Oh. But that means combat will never interrupt rest, since a 600-round combat is unheard of. Why list it at all then?
Mike Mearls @mikemearls 10:05 AM - 10 July 2014
@sevensideddie there could be cases where it's valid - fight starts, now you need to leave the dungeon
The Seven-Sided Die @sevensideddie 10:07 AM - 10 July 2014
@mikemearls Ah, so it’s meant to be fairly rare, more “we’ve given up resting for now,” not just attacks on the camp.
Mike Mearls @mikemearls 10:08 AM - 10 July 2014