* There are people with generally higher energy levels, personally I suspect it's mostly about the dopamin/noradrenalin system. * People can choose different strategies how to approach their work. Some are very good at controlling the amount of attention they put into tasks. They might spend lower amounts of energy per task compared with other people, thus having more overall energy to spend. * The types of task you do. I can sit in meetings all day. Draining but not exhausting. However, difficult programming and data analysis, writing acamedic texts, having actually creative thoughts? 4-6 hours per day, tops. * Stimulants. Lots of them. Again, mostly about dopamin/noradrenalin. Amphetamines, modafinil, nicotine, caffeine, .... There's a plethora to choose from, especially if you live in the US.