How do I keep brisket moist overnight?

Don't keep it. Set your alarm clock and start smoking in time to finish for dinner time. Once you get the firebox going you can go back to bed and set an alarm for 2 hours later. Unless you have done this a few times then keeping the firebox going properly is not an automatic. You can finish most brisket in 6-8 hours. The brisket should already be double wrapped in tin foil.

If you're already committed to smoking, you're 90% of the way to another option entirely, and that's to make a cured, smoked meat product, which has sufficient salt and smoke to act as a barrier to bacterial growth. Do follow a recipe, for example: http://amazingribs.com/recipes/beef/close_to_katzs_home_made_pastrami.html

At that point, there's no food safety concern about leaving the meat at room temperature.

Smoking a brisket until it hits around 160F and then finishing it in the oven is a common way to cook brisket. Meat doesn't benefit much from further smoking after it hits 160F or so, and the oven is much less finicky than a grill or meat smoker. For reference, here is a good description of smoking brisket in a charcoal smoker and then finishing in the oven.

When it's time to take the meat out of the smoker, normal practice is to wrap it thoroughly in foil and then move it to the oven right away. Once the meat hits the final temperature (205F or so), you can hold it for hours before serving it, as long as you keep it warm. So, what you want to do is to stretch out the oven portion of the process so that you can go to sleep.

I'd do something like this:

1. Smoke the brisket to an internal temperature of 160F or so.
2. Wrap it thoroughly in heavy-duty foil. You want to prevent any liquid from being able to leak out.
3. Transfer the roast to an oven set to 225F or even 210F-215F if the oven will go that low.
4. Go to sleep.

With the oven set so low, the meat will take a long time to reach its final temperature. This is a good thing because brisket benefits from being cooked low & slow. And in the worst case, the meat won't be able to get any hotter than the oven temperature.

The next morning, you can check the meat temperature and make any final adjustments. If the meat isn't cooking fast enough, you can turn up the oven. Once the meat reaches temperature, you have a couple of options:

• Remove the roast from the oven, and (leaving it in foil) wrap it in towels and put it into a small insulated cooler. It will lose very little heat this way, and it'll have even more time for collagen to melt.
• Turn the oven down to its "keep warm setting" (hopefully 170F-180F) and hold the meat that way.

However you keep it warm, pull it out at some point to let the meat get proper resting period. When you finally open that foil, you'll probably find the meat is resting in a pool of wonderful smokey beefy smelling liquid. You can pour that back over the sliced meat, or incorporate it into a finishing sauce of some kind.

I've let Brisket rest for up to 5hrs with it wrapped in a cooler with towels.. Still too hot to handle with bare hands and super tender and moist. No bacteria growth ect.. Best brisket I ever made and I pulled at 203.We alive so there's that lol

I have started my briskets and finished in the oven for many years. I have never had an issue with tenderness or flavor. The brisket is amazing.. I start on the smoker , once its about 140-160 I wrap, set oven to 215 and when I wake up I take it out , let it rest a while then I cut my burnt ends off and finish them in the oven. Never had an issue. Always make amazing briskets. Time can be an issue when staring a brisket at 9 pm..So this is the method that works best for my time.