It's normal to feel our sleepiest during two particular times of day -- one of which is before bedtime, while the other occurs when many of us office-dwellers happen to be desk-side. Circadian rhythms (the physiological 24-hour cycle) dictate that it's totally normal for humans to be  drowsy around seven hours after waking up. So, say you wake up at 7 a.m. and arrive at the office around 9 a.m. By the time 2 p.m. rolls around, you'll naturally be a bit sleepy. 
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