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The description of a long rest says:

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.

A recent Sage Advice, while discussing whether elves need 8 hours for a long rest, also clarifies that a long rest need not necessarily include sleep, and that the only activity limited to 2 hours is the standing watch:

A long rest is a period of relaxation that is at least 8 hours long. It can contain sleep, reading, talking, eating, and other restful activity. Standing watch is even possible during it, but for no more than 2 hours; maintaining heightened vigilance any longer than that isn’t restful. In short, a long rest and sleep aren’t the same thing; you can sleep when you’re not taking a long rest, and you can take a long rest and not sleep.

Presumably, though, most humanoids need to sleep (and elves need to trance) to avoid exhaustion. Yet I cannot find any rules relating to applying levels of exhaustion to characters who neglect (or are unable) to sleep. I don't think the forced march rules apply here, as long as the characters limit travel/adventuring to 8 hours/day and spend the rest of the day doing downtime activities and resting.

I am thinking primarily of a situation where hallucinations, nightmares, or a noisy environment prevent a character from sleeping when he or she attempts to do so.

What/where are the rules that require a character to eventually sleep or trance? And what are the penalties for not doing so. The sage advice says sleep is independent of rest, so what is the penalty for not sleeping if the character has taken a full long rest consisting of only light activity.

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There are no rules that enforce sleep, but a mechanic of penalizing lack of sleep exists in the PHB.

There appear to be no rules in the PHB regarding just how much a character needs to sleep. However, it can be inferred from the description of the Trance feature that non-Elf creatures require up to 8 hours of sleep. Nothing specifically states what happens if the time spent sleeping is less than that, though. There is only a suggestion for skipping sleep entirely:

In the description of Constitution (PHB page 177) it says that the DM may request a character to roll a Constitution save if it tries to "go without sleep".

Whether the same thing could be done for sleeping a fraction of the full 8 hours is never mentioned, and probably left to the discretion of the DM.


Suggestions for house-ruling required sleep:

Seeing as Elves seem to be the only exception among the playable races of the standard edition, we may presume every other race behaves similarly to humans. And we know grown humans require between 7 and 9 hours of sleep (though this is a point of continued research and debate). The average of that is 8 hours, which neatly corresponds to the "8 hours for fully rested" principle inferred from the Trance feature.

One could then assume that a long rest of 8 hours of which a non-Elf character spends 2 hours standing watch and 6 hours sleeping will successfully complete the long rest, but leave the character a bit tired and wanting more sleep. One could also assume that not sleeping at all, while still technically a long rest, will leave the target exhausted. How to transfer this into rules and what to do with the states in-between is at the discretion of the DM. Usually the DM just lets you feel rested unless a special circumstance comes up. If you want sleep to matter in your campaign, though, I would suggest making a house rule. It could be something like:

If the character performed any strenuous activity during the day, he would require 8 hours of sleep to feel fully rested. Sleeping less than required leaves the character tired. If the character sleeps less than 8 hours a day for a number of consecutive days, the exhaustion accumulates. Once the character accumulated a total of 8 hours of loss of sleep, the character rolls a Constitution save. On a failed save, the character suffers one level of Exhaustion. Spending at least 8 hours sleeping or spending a day in light activity resets the counter.

This would mean that, if you were to spend only 6 hours sleeping every day during heavy adventuring, after the fourth day you'd have to start rolling Constitution saves (2 hours of sleep lost each day * 4 days = 8 hours of sleep lost). For a 7 hour sleep regime you'll be safe for 8 days. After you finally get your 8 hour sleep, you feel like you've caught up and that you're well rested. Another way to reset the sleep loss counter is simply traveling (without a forced march) and/or only engaging in other non-strenuous (primarily non-combat) activities during the day.

Elf's racial feat would then have an actual purpose. The same rule would apply, except for requiring only 4 hours of trance. Leaving the rest of the rule unchanged would then mean that an Elf would have to spend two consecutive nights not sleeping at all before he's forced to roll any Constitution saves (4 hours of sleep lost each day * 2 days = 8 hours sleep lost), and to reset the counter he'd just have to take a 4 hour snooze.

If counting missed hours of sleep seems like a tedious task, a simpler rule might suffice:

If a character slept less than their required amount (4 hours for Elves, 8 hours for the rest), they roll a Constitution save. On a failed save, the character suffers one level of Exhaustion. The DC for the save is reduced by 1 for each hour that the character spent sleeping.

As a DM you would have to decide what the base DC is. Here's a handy chart for a range of difficulties that you can use depending on the situation. For instance, the check should be easy at first, but gradually (point by point) get harder as the adventurers continue to miss on their sleep.

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    \$\begingroup\$ I like your houserule. It reflects the rules for constitution saves on the PHB that just says that the DM can demand constitution saves caused by lack of sleep. The DM seems to be freely allowed when to apply them. A houserule as mentioned by you would be an easily solution if the group would like to have it matter. \$\endgroup\$ – Aldaris Oct 1 '15 at 7:32
  • \$\begingroup\$ Have you used this houserule in games you have run? \$\endgroup\$ – Wibbs Oct 1 '15 at 7:37
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    \$\begingroup\$ @Wibbs there was no need for it so far, as players instinctively tried to make sure to get enough sleep, and I've never pushed the situation on them. But as I was writing this I did start considering doing some... experimentation. (evil DM laugh) I wouldn't force them into skipping sleep, though, as that probably wouldn't feel fun. But something like a cursed weapon that constantly whispers to the character and keeps them awake at night... The players will suddenly be very motivated to get rid of that curse :D \$\endgroup\$ – DaFluid Oct 1 '15 at 7:50
  • \$\begingroup\$ This answer is outdated due to errata. \$\endgroup\$ – David Coffron Aug 1 '18 at 17:09
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It's first worth mentioning that the Sage Advice you cite is now obsolete; errata have been released which contain a correction so that long rest does now require sleep as a general rule:

A long rest is a period of extended downtime, at least 8 hours long, during which a character sleeps for at least 6 hours and performs no more than 2 hours of light activity, such as reading, talking, eating, or standing watch.

The Basic Rules (which still give the original version) and the PHB (which is updated in newer printings and on D&D Beyond — payment required) don't say exactly what happens if you continue to not sleep, other than not getting the benefit of a long rest, but Xanathar's Guide adds rules for this: DC 10 constitution saving throw to avoid a level of exhaustion after 24 hours of being awake, and the DC goes up by 5 for each additional 24 hours.

For some reason, there's no level of exhaustion between "so tired you can't move" and "completely dead". It seems like at some point "you fall asleep" would be on the chart, but, nope. Death it is.

With that in mind, a Con 10 character without proficiency in that save has a 16.5% chance of being fine after 48 hours, but only a 0.82% chance of feeling normal after 72. (Of course, a character who made it through the first 48 has a 5% chance of one more night where everything seems fine.) After that, though, the DC is impossible, so it's just a matter of counting down to sudden, permanent sleep.

You'd think a level 20 barbarian with 20 Con is would be much better at ignoring the pesky limitations of mortal flesh — and indeed the first night doesn't even require coffee, and the second presents just a 15% risk of getting a level of exhaustion. But as the DCs go up to impossible levels quickly, outside the bounds of 5E's math, everything goes downhill suddenly. Even with a streak of good rolls (early on, where they even matter), the eleventh night is certain death for even the most hearty hero.

Apparently sleep is significantly more important in D&D than in the real world; in 1965 a high school student stayed up for 11 days, with no apparent long-term harm. And there is no indication that this student was particularly proficient in Con saves, or even had classes levels (beyond those available in high school). Scientists have monitored subjects kept awake for 8-10 days, also with no harm that couldn't be fixed by "one or two nights of recovery sleep" — which is much faster than 5E's slow recovery of one exhaustion level per night.

I find this somewhat disappointing: I know that D&D is not a good physics simulator, but I expected it to be pretty accurate on the subject of all-nighters.

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    \$\begingroup\$ Heh, college students who play D&D 5e will probably appreciate this nod to the problem of pulling too many all nighters or cramming for exams. :) \$\endgroup\$ – KorvinStarmast Nov 16 '17 at 21:39
  • \$\begingroup\$ “Not a good physics simulator” - it also isn’t very good at physiology \$\endgroup\$ – Dale M Nov 16 '17 at 22:39
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    \$\begingroup\$ I know you can't die from lack of sleep but you can die (quite easily) from trying to adventure and fight while not having enough sleep. But it looks to me like these rules are just meant to convince your players to let their character sleep. \$\endgroup\$ – Michael Gorman Nov 17 '17 at 14:17
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    \$\begingroup\$ While D&D is not a meant to be a simulation the longest I have ever remained awake and active was 64 hours (only alcohol and caffeine before anyone asks). My cognition started to seriously degrade (as opposed to just very tired) between 50 and 60 hours (very much an estimate) where I also began to have minor but potentially problematic hallucinations (such as a rock wall appearing to be made of bricks despite me knowing it wasn't). At 64 hours I got home and fell into bed and had a looooong rest. Not very dissimilar to the rules actually, though obviously anecdotal. I don't recommend it. \$\endgroup\$ – Protonflux Nov 17 '17 at 15:01
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    \$\begingroup\$ Fun fact, Guinness World Records stopped tracking the longest anyone goes without sleep because it resulted in several deaths and other serious issues for persons attempting it. That high schooler is an odd exception to the general rule that skipping sleep for too long is very hazardous to your health. \$\endgroup\$ – Pyrotechnical Aug 1 '18 at 17:47
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Existing mechanics

There is a specific mechanism in D&D 5e for going without sleep; indeed, it is the same mechanism for every other action in the game (PHB p.6 - How to Play):

  1. The DM describes the environment.
  2. The players describe what they want to do.
  3. The DM narrates the results of the adventurers’ actions.

As you have pointed out, you can get the benefits of a long rest without sleeping.

You have also touched on the other mechanic at work here: exhaustion. Don't get bogged down in the game mechanics - start with English: someone who is sleep deprived will eventually become exhausted; when they do you apply the mechanics.

So, how long before this matters?

Well, when making this sort of judgement, look to the rules, look to the real world and look to personal experience.

The rules suggest (PHB p.177) that a Constitution check is appropriate to "go without sleep". They also give rules which I think can be easily adapted for how Forced Marches trigger exhaustion (p.181) (DC10+1 per hour) and how lack of Food and Water do (p.185); the food method is not similar sleep deprivation but the water one looks adaptable (DC15 for 1/2 requirement or automatic for less and doubled if already exhausted).

In the real world "Human sleep needs vary by age and amongst individuals, and sleep is considered to be adequate when there is no daytime sleepiness or dysfunction." and an adult human should get about 7-9 hours sleep per day. Note: per day - not per night, bimodal and siesta style sleep works for humans and our monophasic pattern appears to be a product of artificial illumination dating from the industrial revolution. Military rosters have traditionally been based on an 8 hours on 4 hours off roster which can be kept up more or less indefinitely. Of relevance the longest non-pathological voluntary period of sleep deprivation is 11 days and there are no recorded instances of a human dying from sleep deprivation: eventually you get so tired you can sleep even in the presence of stimulus.

Personally, I have had nights where I have slept less than 4 hours and functioned OK the next day. I have also had periods of up to 48 hours without sleep and have felt like s*#t; Level 1 exhaustion perhaps? I personally know several people who have had stints of 72-84 hours without sleep during military service and they have reported severely impaired cognition, hand-eye coordination and an overwhelming desire for sleep; Level 3 exhaustion perhaps.

Putting it together

Proposed rule:

Sleep Deprivation

A character can go without sleep (or meditation) for 1 day with no ill-effects. For each additional day of sleep deprivation, the character must make a Constitution saving throw at the end of the day. The DC is 15 for each day without sleep. On a failed saving throw, a character suffers one level of exhaustion (see appendix A). A character cannot reach exhaustion level 6 (dead) through sleep deprivation; instead they become unconscious for 8 hours. A character cannot recover exhaustion from a long rest while sleep deprived unless that rest includes at least 4 hours of sleep or 2 hours of meditation.

I experimented with a few different options for the DC and if it should be flat or go up by 1 per day. I remembered to include disadvantage on the save once you reach level 3 exhaustion.

A DC of 15 has a median for a character with a Constitution of 10 to reach level 5 after 6-7 days without sleep. A 20 Constitution character pushes this out to 9-10 which I think is a nice gap between the average and the best.

Using an ascending DC doesn't change the median much but it squashes in the outliers. A 10 Constitution character has a 90th percentile of 8 days and a 99th percentile of 9 for level 5 exhaustion for an ascending DC compared to 9 and 12 for a flat DC.

I like the flat DC because it gives a (remote) chance of superhuman feats of endurance and is easier on the record keeping.

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  • \$\begingroup\$ FWIW, I think your house rule is better than the Xanathar's rule. Or perhaps a DC of 10 the first night and 15 thereafter. \$\endgroup\$ – mattdm Nov 16 '17 at 22:21
  • \$\begingroup\$ People have died from lack of sleep, particularly in the case of fatal familial insomnia. \$\endgroup\$ – Ettina Kitten Aug 1 at 0:36
  • \$\begingroup\$ @EttinaKitten Fatal familial insomnia is a prion-caused pathological disease, it isn’t comparable with healthy people being sleep deprived. \$\endgroup\$ – Dale M Aug 1 at 1:04
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In the long run, death.

In the short run, just levels of exhaustion that tick up each day.

With the release of Xanathar's Guide to Everything, there are more formal (optional) rules on how sleep works (p. 78):

Going Without a Long Rest

A long rest is never mandatory, but going without sleep does have its consequences. If you want to account for the effects of sleep deprivation on characters and creatures, use these rules. Whenever you end a 24-hour period without finishing a long rest, you must succeed on a DC 10 Constitution saving throw or suffer one level of exhaustion. It becomes harder to fight off exhaustion if you stay awake for multiple days. After the first 24 hours, the DC increases by 5 for each consecutive 24-hour period without a long rest. The DC resets to 10 when you finish a long rest.

Additionally, if someone sleeps in medium or heavy armor, they receive reduced benefits from the long rest (p. 79):

Sleeping in Armor

When you finish a long rest during which you slept in medium or heavy armor, you regain only one quarter of your spent Hit Dice (minimum of one die). If you have any levels of exhaustion, the rest doesn't reduce your exhaustion level.

Also, as pointed out in a comment, both of these rules are designed to assist Dungeon Masters in making a game run smoothly. They are not set in stone and they do not have to be followed. From the intro to Chapter 2 (p. 77):

The material in this chapter is meant to make your life easier. Ignore anything you find here that doesn't help you, and don't hesitate to customize the things that you do use. The game's rules exist to serve you and the games you run. As always, make them your own.

The problems that these rules can potentially pose are specifically addressed in another answer to this question.

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    \$\begingroup\$ You should note that these are optional ways of handling the situation \$\endgroup\$ – David Coffron Aug 1 '18 at 17:39
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    \$\begingroup\$ I'm glad to see that Xanthar's added optional rules for these situations, but they don't actually address the situation I asked about which was The sage advice says sleep is independent of rest, so what is the penalty for not sleeping if the character has taken a full long rest consisting of only light activity? The context I was asking in was one where characters were plagued with nightmares that made it impossible to sleep restfully. I could impose the "no long rest" penalties, and they seem quite appropriate, but they don't actually address the difference between rest and sleep. \$\endgroup\$ – PurpleVermont Aug 2 '18 at 18:20
  • \$\begingroup\$ I actually think it is quite clear, although I can amend my answer to better clarify. A player needs to simply be doing nothing strenuous to gain the benefits of a Long Rest. That much is made apparent in the Sage Advice. As such, if a player is physically prevented from sleeping, they can still take a long rest as long as what is stopping their sleep is not classified as strenuous. \$\endgroup\$ – Michael Ziluck Aug 6 '18 at 11:54

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