tl;dr the duration of the rest is unchanged, but 4 hours per day is spent in trance instead of 8 in sleepFor those of you who extend the time of rests; how do you square the Elf's Trance ability (they mediate 4 hours a night and get full long rest)?
A breather is a period of downtime, at least 1 hour long, during which a character performs no more than lowkey activity such as reading, talking, eating, drinking or standing watch. If it is interrupted by adventuring activity—fighting, casting spells, marching, or similar—characters must start the rest over to gain any benefit from it. At the end of a breather, characters can spend Hit Dice to regain hit points.
A breather in which characters go on to sleep or trance may be extended into a short rest of about a day. At the end of that rest, characters who prepare spells can change their lists, and features that can refresh at the end of a short rest, do so. Those who rest in comfort and eat and drink, recover one level of exhaustion.
A short rest can be extended into a long rest of around three days. A character must have at least 1 hit point at the start of such a rest to gain its benefits, and must sleep or trance each day. At the end of that rest, characters regain all lost hit points, and they regain spent Hit Dice up to half their total number of them (at least 1); any features they have that can refresh at the end of a long rest, do so. Those who rest in comfort and eat and drink, recover completely from exhaustion.
When characters finish a rest incorporating a given type, they can’t benefit from another rest of that type until time equal to its duration has passed, e.g. characters finishing a short rest can’t benefit from a breather for an hour.
Sleeping and Trancing, and Armor
Characters who sleep need 8 hours to do so, while those who trance need only 4. Warlocks benefiting from Aspect of the Moon can spend 4 hours reading their Book of Shadows instead of sleeping. Characters sleeping or trancing in medium or heavy armor aren’t comfortable. At the end of an uncomfortable rest, characters don’t reduce exhaustion and regain a quarter of their spent Hit Dice instead of half.