Did you just start taking metoprolol? Maybe it lowers you're blood pressure a little too much making you light-headed and then after several hours the bp goes up again. What are you taking it for? I take 12.5mg (not extended release) twice a day for an arrhythmia. Have been taking it for over 2 years. Initially it really slowed my pulse and b.p. but eventually the body seems to get used to it. I didn't get lightheaded just sleepy. Lately, I have noticed that I experience fatigue for a couple of hours in the afternoon after taking my first pill of the day at noon. The doctors just say the metoprolol is doing its job.