I notice that testosterone used to be sort of an "adrenaline fuel". When I was very tired, I would get sleepy for a while, but if I had something to do, it would quickly go away, as though entering "emergency wake mode". Since HRT, though, that "mode" doesn't work very well. But that's probably good, since it helps me fight my tendency to have an unhealthy sleep schedule.
Now, if I'm extenuated, my body TELLS me. As a result, I actually have more energy the next day.
And it adds up to the overall energy boost that HRT provides.