Hello,
I have anemia, so my Iron levels can get pretty low especially if I am doing strenuous exercise (such as Bikram). One of the effects of having low Iron is extreme low energy and tiredness. I am not saying you have this but it might be an option to get a blood test checking your Iron levels. I found that once my Iron is at a normal level my evergy is great, I can practice bikram daily and I don’t feel the need to fall asleep at my desk! 😛
Hope this helps.
Cheers
Ny