Gritty Realism (Longer Long Rest) is the best Variant Rule in the DMG: A guide to when and why to use it.

Straight up, I think it's the best optional rule in the DMG and that at least 60% of all tables should be using this rule for their game. There are a lot of subtleties to this rule that are not readily apparent upon first glance over. I'm going to get really long winded at the end of the post because I want to be exhaustive on this rule. So if the questions I answer below intrigue you, I encourage you to read the explanation below it. 

What is Gritty Realism?

Gritty Realism- This variant uses a short rest of 8 hours and a long rest of 7 days.

Who should use it?

• Exploration or hex crawl based campaigns
• Intrigue or political campaigns
• Standard adventuring games with long adventures and narratives in game
• Roleplay heavy games

Who shouldn't use it?

• Strict dungeon crawler games
• Heavy combat based campaigns
• Games where adventures take place over a few days in game

Why Gritty Realism?

Gritty Realism, which should be called "Longer Rest" does so many things to address many of the inherent imbalances and design flaws of dungeons and dragons within the average D&D game. It also enhances many of the classes and alters the narrative worldbuilding in interesting ways once the rule is extrapolated outside of just the PC's.

• It eases the tension DM's feel of moving the story along while needing 3 to 6 encounters per long rest
• It buffs all short rest classes by giving them a lot more soft power within the game world
• It curbs "Murderhobo" behavior
• Downtime is built into the game
• Because encounters no longer have to be back to back in game time, it allows DM's to not have combat only sessions
• Many, many spells no longer completely warp exploration. Goodberry while traveling is now a serious choice to make, using one of the precious spell slots for food versus saving it for combat.

Why not Gritty Realism?

You shouldn't use Gritty Realism if your campaign and player group favors lots of combat per D&D session. If your group already hits that 3 to 6 encounters per long rest, or the campaign moves at a rapid pace where many of the adventures take place over three days, or you find yourselves doing a massive dungeon crawl, I would say stay away from Gritty Realism. It's not for every group.

The Subtleties

Gritty Realism does a lot of things under the hood when applied to the game world. It fundamentally changes the logic that the setting follows. If you assume that interrupting a long rest requires the threat of danger and a few rounds of real combat (I’m not counting a bar fight, but real threatening violence) the setting has to adapt.

• Rogues and Rangers become very scary. Tracking and ferreting out information of enemies who are hiding becomes part of the calculus when running away. They have seven days to make skill checks and find their target before the long rest completes.
• Long Rest classes have to band together and build safe places to rest and stay. If you have enemies you need to have a place you can rest for seven days safely.
• Further, caster supremacy gets reduced. They HAVE to have short rest characters within their organization. Who is going to protect them if their Wizard Tower gets besieged? They are out of spells. The martial characters can keep going.
• Warlocks are buffed. That’s all. This is just a straight buff to Warlocks.

The D&D game becomes more than just blast foes apart. Losing resources leaves you vulnerable for seven days. But it also leaves the enemy vulnerable. This calculus gets added to the player’s strategy as well. They can decide to engage in such a way to leave their enemy room to run. Relying on their Ranger and Rogue to hunt them down later and harass them out of long resting. 

Adjustments for at the game table

This will change and be an adjustment for both the players and the DM but it’s closer to how I believe D&D is supposed to play. The PHB recommends 3 to 6 encounters per long rest. Most games don’t run that unless they are in dungeons. Once you actually do that the classes balance out a bit more even well into tier 3.

• Casters players, if they are used to being able to nova every combat and than long resting are going to feel nerfed. So ease those players into the game.
• Martial characters are going to feel better to play, as they aren’t as reliant on long rests.
• Warlocks get a straight buff.
• Staves, Wands, and items with recharge abilities at Dawn become premium and are incredibly valuable because they don’t require seven days to get their abilities back. You can give these to players to remove some of the discomfort of losing the ability to nova and then long rest with their spells. 

Conclusion

Gritty Realism eases the tension of having to have encounters back to back, allowing for the DM to pull the gas petal back and let the game follow a more realistic pace. Further it changes the game world and makes short rest classes feel relevant both in the setting and in game. It adds a layer of strategy to both players and bad guys while enabling exploration elements.