I am so tired

I am so tired. This exhaustion comes and goes so suddenly. I can be passing out, exhausted, barely able to move one moment, and suddenly I’m up and raring to go. One moment I’m getting ready to ride my bike across town to buy something to take on another big creative project, and the next moment I can’t find the strength to tie my shoes. Where does this energy come from and where does it go when it leaves me helpless?