I don't just want to know about chemical solutions to the problem either. Anything will do. I'm curious about how people could stay awake and alert during battle when sleep is hard to get. And if you know something about Nazi meth, I won't stop you from sharing.
From the perspective of USAAF aircrew flying over Japan in B-29s, where an operation of regularly in excess of 24 hours duration, Benzedrine was used to keep them awake, while they took Benedryl to sleep after landing.
The RAF also used Benzedrine and similar compounds (referred to by Australian air crew serving in the RAF as Wakey-wakey pills) to keep the crews alert on night operations conducted by Bomber Command. -There are enough anecdotal reports floating around of the benzedrine and seasickness tablets being handed to all crew by the squadron doctor.
In the US military they're usually referred to as "go pills". The US Air Force has historically made them available to pilots with varying levels of regulation. In Germany it was called Pervitin, and was usually (though not always) mixed with chocolate and distributed to soldiers, and because of it's association with air and tank crews, came to be known as "Panzer" or "Fliegerschokolade". It was used in Japan as well, in military and civilian contexts.
As for non-chemical solutions, I don't really know. Obviously there are night watches and pickets, but that's not so much a solution to staying awake as it is being able to wake everyone in case of attack.