I've tried to explain this problem to a doctor but she didn't believe me. Sunlight makes me sleepy. When I am in a place with natural lighting I am very tired, I can't focus, I have trouble keeping my eyes open, I'm disoriented, my mind is foggy, I have every symptom of severe fatigue. When I move into a room with little or no natural light, my mind clears and I wake up. I am alert and focused at night or in moderate artficial lighting. Strong fluorescent or full spectrum lights cause the sleepiness as well, but not as severely as sunlight. Even if I have had a full night's sleep and have only been awake for an hour or two, I can fall asleep in a sunny room. This doesn't happen in a dark or artificially lit room.
I know this isn't caused by sleep deprivation, depression, caffeine or drug withdrawal, or any of the normal causes of fatigue because I am only affected in sunlight, never in dark or artificial light. As soon as I walk into a sunny room I get sleepy and disoriented, but when I go back into a dark or artificially lit room I wake up again. My fatigue is being caused by sunlight - what I need to know is what medical condition this is and if there's any way to treat it.