Honestly I've never figured it out either. I also have a 3 year old so I have to function. I will have a small cup of coffee when I start to fall asleep. Sometimes I will randomly take a walk. It makes me push through the exhaustion.
My doctor told me stretching and exercises help. I have the fatigue spells too when it gets close to dinner time I want to sleep but I'm the one who makes dinner.
I usually take our dog for a walk. I walk the dog more now that I'm pregnant to keep moving and active. Once I stop moving I will fall asleep. My 3 yr old seems to think we get up every morning at 5am so that's killing me.
Mine was an iron supplement from my dr it worked really well for my sluggish moods :)