It’s dehydration, I have twenty years experience with millions of visitors, near Moab Utah. But don’t take my word for it, theMayo Clinic lists "“Tiredness” before “Thirst” in it’s symptoms of dehydration.
From the U.S. Army:
EFFECTS OF WATER LOSS
The body has a small reserve of water and can lose some without any effects. After a loss of about 2 quarts (which represents about 2.5 to 3.0 percent of body weight), effectiveness is impaired. Soldiers/marines may begin to stumble, become fatigued and unable to concentrate clearly, and develop headaches. Thirst will be present but not overpowering. So unless well trained, or reminded or goaded to drink, troops may not replace the water loss.
Does not say that the sun causes such dehydration. I know the sunshine fatigues me and does so if I haven’t sweated much. I’ve sweated much more during some exercise routines and was not dehydrated to that extent and not drained to the extent that the sun drains me. Sorry. I don’t buy that.
It’s certainly an easy enough experiment to conduct on yourself. Drink enough water that your pee comes out clear and see how you feel.
I feel it’s more of a comfort thing, depending on the day. The days when I go lay in the sun (because I can’t spell the correct form of lye) are almost invariably dryish (low humidity), so the air is warm enough but not too hot, surfaces are warm to the touch, and it’s almost like you can feel the sun rays that are all warm and soothing. Goodness knows I don’t want to move away from a sunbeam like that - I’m a cat at heart. 
I think it has something do w/your horoscope signs, I’m not an expert but I’ve experienced that diff signs react differently to the sun… I’m a Cancerian, a sign ruled by the Moon. I feel extremely fatigued and drained after 4-5 hours in the sun, (while keeping hydrated by drinking water, i.e. water rafting trips, driving during peak hours in the sun)… I hate being out in the sun and I love the evening/night times…
My husband, a Capricorn - ruled by Saturn, on the other hand LOVES the sun and he can sun bathe for hours w/o feeling tired/fatigued. If he’s indoors for too long, he starts feeling tired and depressed, as if Sun actually gives him some more energy… One of my friends, a Leo, ruled by the Sun, also LOVES the sun…
I’m not an astrologist/scientist/doctor but just a thought…
My vote is on being too damned hot. If you’re sitting indoors with no air conditioning and it’s over ninety degrees and you’ve been properly hydrating, you still fill exhausted and you have not been in the sun.
I am a doctor, so you can trust my WAG on this. 
But first I must say I reject the dehydration theory wholeheartedly. Just like the poisonousness of “out dated” food, people drastically over estimate the occurance of dehydration. I blame it on the bottled water sellers. People who are not ill operate with a copiously abundant reserve of body water such that losing a little by sweating is literally a drop in the bucket. You will concentrate your urine to account for most of the loss anyway. Any “negstive” balance after that is, in any nonextraordinary circumstance, negligible. I really have to bite my lip when people start blaming all kinds of things from headaches to blurry vision to a blase mood on “dehydration”. It just doesn’t wash. Sorry to go on about it but it’s one of my little peeves.
I’d say the real answer is a combination of 3 and 4 from the list, hands down.
Astrologist and scientist/doctor should not be listed together. Astrology is hogwash.
I’m not a doctor - as will doubtless be obvious - but I checked Wikipedia to see what the effects of Vitamin D overdose would be. Somewhere in there it seems to say that it would be pretty difficult for excessive sunlight exposure to cause a Vitamin D overdose. During extended sunlight exposure, the body will naturally start ‘degrading’ Vitamin D precursors, probably to prevent such an overdose. Very good thing, it sounds like.
I’ve experienced the ‘wiped out’ feeling myself. I suppose it could be simple overheating, so maybe a cold shower might be at least part of the treatment. …I still wonder if there could be various other chemical things going on, not directly part of the Vitamin D thing, but in there somewhere. And it seems that maybe only some people are particularly sensitive.
There’s a research project in here somewhere, maybe.
My WAG is that if you’re “laying” out in the sun, you are probably a female of a species that lays eggs. So possibly the egg-laying process itself drains you of energy . . . or perhaps it’s all those feathers.
It’s all explained by Robin Wood’s Theory of Cat Gravity:
http://www.robinwood.com/Catalog/Prints/PrintPages/CatGravPoster.html
Seems like this thread is well into WAG territory. I can participate on that playing field.
Know how when it’s cold, your muscles tense up and shiver? This is an autonomic response that burns enjerny to produce heat.
When you’re hot, conversely, your body does the opposite autonomic response. Your muscles will relax, in order to burn less enjerny, to minimize the production of heat. This is one of the body’s temperature-controlling responses.
Heat is well known to be relaxing. It is a well known effect of lounging in a hot tub or steam bath. Heat is used to relieve spasms. Lying out in the sun will have the same effect. I think this effect is independent of hydration or whether it’s dry heat or wet heat. The sensation of having your muscles go limp like that gives rise to the perception of being drained of enjerny.
ETA: moriah refers to the same phenomenon in her post (#27 above).
(Missed edit winder.)
ETA2: Are y’all sure the correct present-progressive form is “lying” and not “laying”? Lying is something that some people on the island of truth-tellers and liars do. (If I mentioned where that really happens, I’d get mod-noted for political snark in GQ.)
Raymond Smullyan includes a logical puzzle in his book “What Is The Name of This Book?” that turns on this. It seems that our visitor to the Island encounters a native, lying lazily in the sun. Visitor asks some questions, gets some answers, and we are given some puzzle to solve. Turns out, the solution depends on the information given in the problem statement, taken literally at face value: The native is lying in the sun. In other words, whatever he said was a lie.
I think it’s simple inertia, really. I spent all day lying around a hospital room today. I wasn’t the patient, my husband was. He’s feeling fine, just killing time until they release him tomorrow when the Attending comes back from the holiday weekend. So I wasn’t particularly stressed, physically or emotionally. I just sat or laid in a chair next to him, chatting, watching him snooze (he got no sleep last night), watching TV, reading a book…
I’m wiped. Seriously weary. I couldn’t have burned more than 300 calories off basic life support metabolism today, and I feel like I haven’t felt since…well, since the last time I lounged around in the sun all day. Certainly much more tired than when I spend all day running around, cleaning house, driving, working, picking kids up from school, doing paperwork…
My WAG is that, when we’re mostly immobile, our bodies ratchet the ol’ metabolism down a notch to conserve energy. It’s like my laptop going on standby. Since most of us are never quite as still as long as we are when we sunbathe, we erroneously correlate it with the sun.
Since so many people note that exercise gives them energy, why would we be surprised that sloth saps energy, in the subjective sense?
-nurse here, several times when my wife and three teens have spent more than a couple of hours in the sun, like, at the beach or while camping, fishing and or hiking but-not always with a lot of activity, we have been “sick. This was always later in day after sun all with same symptoms almost like " a tired/hot/toxic/weird/grouchy/headachey” post flu syndrome? I know dehydration not part of, i always played with the idea of “radiation poisoning”- ie…sun vs body/skin, always better next morning.
It’s not simply inertia or any of the other WAG reasons posted in this thread. I know what it isn’t, but don’t know what it is. Years ago I competed in outdoor swim meets. Just sitting in the hot sun depleted me. By the time it was my turn to swim, I was exhausted. I would also get a headache. The water refreshed me somewhat. It’s not dehydration as I did not sweat much just sitting. I sweat a lot more running or playing tennis. Funny that. I can run and play tennis and not feel so depleted as just sitting or lying in the sunshine.