Think rationally for 59 minutes then start thinking irrationally until you feel alert again. If you are unable to think irrationally on your own then watch some videos about flat earth or anti vaccination.
Seriously though, my first thought is that you are not getting enough exercise. It is normal to get tired and sleepy after intense thinking, after all it does use up a huge amount of energy. So maybe you need to make sure to get enough energy into ur body before thinking and a bit of quick exercise should help too.
Learn to witness your own thoughts and to look curiously into the goings on of your mental processes with a kind of amused state of non attachment.... learn to breathe... and become breath... it is a trans-rational thing I'm talking about... that and exercise and proper diet and proper sleep may help with the sleepiness thing.