It's hard to describe, but I can quite easily get "into the flow". This is where I focus on a single project, time cease to exist and I just become one with the code. I can focus on coding for hours and hours without break. In fact, lunch and toilet breaks are irritating distractions. I can go to sleep at night uneasy about something I'm not exactly sure about and suddenly the solution strikes me when I'm half awake in the shower next morning.
I don't use much caffeine, a bit of coffee in the morning, rarely in the afternoon – however I do know that coffee and alcohol makes my condition worse.
It's totally engulfing and extremely productive. This _is_ the mythical man month that people talk about. Only. For me it takes a toll. It's not about any underlying stress like "I won't make it in time". My work is luckily such that I don't have much pressure. It's being "in the zone" that is the problem.
The "flow state". I love it yet mustn't overdo it.