Why do I get immensely physically tired from mental strain

No.12696093 ViewReplyOriginalReport
For example, one day I took a 7 hour test and also 150mg Vyvanse, more than I had ever taken before. After finishing the test, I was so tired I immediately took a 3 hour nap and still fell asleep perfectly fine later than night. I also lose/gain weight depending on the frequency I interact with people.