Will a thermos keep coffee hot overnight?
Some of them will and some of them will not. The best thermoses are usually vacuum insulated and made of stainless steel and can keep coffee warm for up to 24 hours. Most thermoses do not reach the 24 hour mark. A common range for most insulated thermoses is between 5 to 12 hours. The cold retention on thermoses will typically be longer than the heat retention.  You can help your steel thermos retention out by priming it. Fill it with hot water for about 10 minutes to warm up the inside of your thermos before pouring your coffee in there. If this is not done, your drink can cool down a few degrees when you pour it into the thermos. Priming is not necessary for cold drinks, but if you are switching from a hot liquid to a cold liquid priming is a good idea or your beverage will warm up when you pour it into the thermos. You can prime your thermos for cold drinks using cold water or ice.