The creators of Dungeons & Dragons and other tabletop games have to come up with rules for everything, ranging from gravity to how fast a shark can swim against a current. There are limitless variables that can come into play with these rules, especially when you add magic to mix, so it should come as no surprise that people have found ways to cheat the system.
The current edition of Dungeons & Dragons has fewer exploits in its rules than previous editions, which has been helped by the presence of social media and the ease in which players can contact the developers of the game. All it takes is a tweet to shut down a rules loophole, which means that it's harder for people to abuse the system.
There are still a few ways to game the system in your favor and one of them involves combining the powers of a sorcerer and a warlock into a class that is fueled by the purest of Red Bull. We are talking about the "Coffeelock" character build, who benefits the most from a lack of sleep.
How It Works
The basic premise of the Coffeelock caster involves combining the Flexible Casting power of a sorcerer with the Pact Magic power of a warlock.
Flexible Casting allows a sorcerer to use sorcery points to create additional spell slots, which can exceed the maximum number of spells they can normally possess (so long as they have the sorcery points to buy them). Flexible Casting also allows the player to convert warlock spell slots into sorcery points, which can then be turned into sorcerer spell slots. Normally, a sorcerer can only regain spell slots by taking a long rest, but warlocks regain them from a short rest.
Rules As Written
In order to reach the Coffeelock build, the player needs two levels of sorcerer in order to gain Flexible Casting (through Font of Magic) and take at least one level in warlock in order to gain Pact Magic.
From the Player's Handbook, page 101.
FLEXIBLE CASTING
You can use your sorcery points to gain additional spell slots. or sacrifice spell slots to gain additional sorcery points.
Creating Spell slots. You can transform unexpended sorcery points into one spell slot as a bonus action on your turn.
Any spell slot you create with this feature vanishes when you finish a long rest.
Converting a Spell Slot to Sorcery Points. As a bonus action on your turn, you can expend one spell slot and gain a number of sorcery points equal to the slot's level.
Player's Handbook, page 107.
Pact Magic
To cast one of your warlock spells of 1st level or higher, you must expend a spell slot. You regain all expended spell slots when you finish a short or long rest.
When the player takes a short rest, they regain their warlock spell slots. These spell slots can be converted into sorcery points using Flexible Casting, which can then be converted into sorcerer spell slots using the same ability. They can then take another short rest... and so on. This means that the player can create bonus spell slots that they can keep until needing to rest, giving them access to more spells than they would normally possess.
A line of errata was added in which curbs this build slightly, as the spell slots vanish when the player takes a long rest (and an optional rule was added that adds penalties for not taking one, see below). Even if the player cannot give themselves infinite spell slots, they can still gain a lot of additional spell slots in exchange for a night without sleep.
Jeremy Crawford (the lead rules designer at Wizards of the Coast) confirmed that this trick can work on Twitter, though he later left a more ambiguous answer a few years later as to whether Flexible Magic and Pact Magic could be combined.
How The DM Can Screw You
The simplest way for a dungeon master to put a stop to the Coffelock build is to prevent a player from using the multiclassing rules. In the current edition of Dungeons & Dragons, multiclassing is considered an optional rule and a dungeon master can refuse to use them in their game.
Xanathar's Guide to Everything included an optional rule that dungeon masters can use to penalize players for skipping a long rest. According to these rules, a player who goes a 24-hour period of time without taking a long rest has to make a Constitution saving throw of 10 or suffer a level of exhaustion. This saving throw becomes more difficult the longer the character goes without sleep and the effects of exhaustion can be debilitating.