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?
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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: