the weed acts on the brain! while physical stamina does play a certain part, you may not feel tired and be able to move around and not feel tired, the brain is going to be sluggish depending on lots of factors of the smoking session. Since MJ acts on the brain it is what gets worn out, you could exercise every day, then smoke more than your used to, and bam, your down for the count. And i dont even think that being mentally sharp would help any, its just a dont smoke weed that will make you sleepy, if you dont want to get tired, aka before work/some fun activity. Also, try eating a good 30-60 minutes before smoking, not only will it reduce your munchies, but the food will have begun to be turned into energy for your body, and will help your brain get the necessary amount of calories to stay awake.