Are you anemic? Women tend to have iron deficiency, and thus can't do things like donate blood. Oftentimes it's not a problem, but people with anemia can become fatigued much faster because not enough oxygen is getting to the muscles and brain (the brain is a major consumer of oxygen). If the brain's not getting what it needs, it will lead to drowsiness and trouble thinking straight. You can get your iron level tested at any blood center to learn if this is a problem; maybe taking iron pills or changing your diet would help. Also, avoid sleep deprivation and caffeine 6 hrs before bedtime--caffeine can affect a good night's rest!