Brisket is a tough cut of meat and requires a long cook time to render connective tissues and fat. Usually smoke times are around 11/2 hours per pound of meat; Meaning a small brisket of 8 lbs could take 12+ hours to finish.
After smoking for a such a long time, most people want to dig in and eat right away. However, doing so will entirely ruin the brisket and render it dry - the exact opposite of your goal.
You should rest your brisket down to slicing temperatures of 150-160F internal. This way, the rendered collagen has had enough time to gelatinize. This resting process will typically take around 2 hours.
Resting simply means to allow the meats internal temperature to come down.
As most people know, brisket is a tough cut of meat and in order to break down these tough connective tissues (like collagen), you need to cook the meat low and slow for an extended period of time.
Note: If you were to do the opposite - ie. sear hot and fast, the temperature would certainly reach 200°F+ but the brisket wouldn't be tender.
The connective tissues inside of brisket won't begin to breakdown until 140°F; The collagen also begins to melt at around 160°F.
Brisket then needs to spend several hours under low and slow heat during this 160-205°F+ window to break down these connective tissues.
During this period, the collagen will render into a gelatin. Gelatin is hydroscopic and will absorb up to ten times its weight in liquid.
Meaning, the moisture that is expelled by the protein fibers needs time to redistribute throughout the brisket and be absorbed by the gelatin.
If you were to simply slice into the brisket after its reached tenderness, the juices wouldn't have enough time to setup - or gelatinize - and they would run out of the brisket.
The result? A dry brisket.
After smoking your brisket, you should allow the meat to rest. The goal is for the brisket to come down to slicing temperatures of around 160°F.
This process of resting from a 195-205°F finishing temperature, down to a 160°F slicing temperature takes roughly 2 hours.
Meaning, your minimum rest time is 2 hours.
However, I'd strongly suggest smoking your brisket the day before. It makes every part of the brisket smoking process that much easier.
Mainly though, you never have to time your cook.
My last 8 lb brisket took me 12 hours to smoke (pictured below). If I were to rest it for 2 hours, that's a 14 hour cook time. Meaning, if my family wants to eat at a reasonable hour, I'd have to wake up at around 3-5 AM.
There's no chance that's happening.
Rather than simply resting your brisket, I'd suggest resting and then holding your brisket.
You'll often hear people say that a long rest is brisket's best kept secret. However, if you really think about it, it's not really a "secret."
When a BBQ joint smokes briskets, it's a borderline requirement that they're smoked a day in advance so that they're ready by 11 am when they open.
I mean think about it, there is no way they're working 3rd shift to smoke briskets - it's just not realistic.
Rather, they're doing a slow rest into an extended hold.
Holding simply means to keep the briskets internal temperature above 140°F.
In a restaurant setting, holding is super important because of food safety. Between 40 - 140°F, food enters the "Danger Zone." In this temperature range, bacteria grow quite rapidly.
These days, almost all my briskets are smoked on a weber kettle. When it comes time for the inevitable stall, I'll usually wait till around 175F internal (which is at the tail end of the stall) and then foil boat the brisket.
From there, I stick the brisket in my electric smoker to finish.
Typical finishing temperatures for brisket are in the 195 - 205F range. However, you should always smoke until probe tenderness.
After probe tenderness checks, you have two options:
*If your oven doesn't go as low as 145F, use the lowest setting. Also, most ovens are inaccurate, I'd be sure to confirm accuracy and the temperature swing (if any).
The reason you're resting until 180F in scenario 1 is because you don't want to overcook the brisket.
The reason you're not resting in scenario 2 is because you want carry over cooking to occur and for all of the tight spots to be resolved via the overnight hold.
Resting is a sort of necessary evil when smoking brisket. Most beginners tend to get antsy and slice too early - only to be met with dry meat.
Rather, rest the brisket for 2 hours - check the internal temperature. If the meat is at around 160F internal, the collagen has gelatinized and you'll be met with moist brisket.
I'd also strongly suggest holding brisket. That way you're not panicked if the brisket finishes too early. Rather, you can look at it being done a day in advance.