My cats love to sit in a sunny patch when the morning sun floods the apartment. They will often groom themselves while there. Every cat I’ve known did this to some extent.
Is there a physiological reason for this behavior? Do they need sun to synthesize vitamin D? I know their body temperature is higher than ours. Does this make them feel chilly at temps that are comfortable for humans? Or is it just a sensual thing - a kitty sauna?
My cat does this as well. She also likes to sit right next to the furnace, clothes just taken out of the dryer and on top of electical equipment, like the monitor.
Is there any reason why it would not be for the same reason I like to sit in a sunny patch on a cool morning? Feels good.
Because they saved the world according to this! 
As mentioned above, I think it’s just for the same reason we would. My cats sit in the sun in summer, sit by the heaters in winter, seem to watch TV all year around, and basically take advantage of any creature comforts available. They still have to eat cat food though. 
In the Irish language, they refer to cats “milking the sun” - ag cru na greine. I always that it was a good description of a cat soaking up the sun.
It is not just a habit of domestic cats. Lions spend a large proportion of their time lying around when they have eaten and are not hunting. It is probably a feline thing.
Cats are very sensual creatures and I suspect it is for the same reason that we sun bathe. It feels good.
It’s not just a feline thing. I have two Basenjis (See www.basenji.com for more info.) that lay in the sun whenever possible. Granted, Basenjis are a bit unusual, but I’ve seen other dogs do this as well. My anecdotal experience is that any short haired dog may engage in ‘sun bathing’. I think it just feels good for them to lay in the sun and relax, same as with us. But they don’t have to worry about getting a sunburn and they seem to have a higher tolerance for heat as long as they are sedentary. I don’t know about feline body temperatures, but I’ve read that dogs tends to have body temps a couple of degrees higher than ours.
In a previous apartment, there was a skylight that the dogs loved. They would lay in the pool of light from it all day, moving every so often as the angle of the sun changed. They would start on one side of the room in the morning and end up on the other side by the afternoon. I always though it would be a fun thing to get a timelapse movie of that.
Our cat not only likes to lay in the sun, but enjoys standing inside lampshades when the lights are on.
One of our dog’s (many!) nicknames is “SunDog” – she’s a German Shepherd/“something fuzzy” (collie/sheltie/golden retriever) mix and she loves to find patches of sun to lay in, both inside and out.
I’ve even seen her laying in full sun on the patio, panting like crazy, with a cool shady patch not 10 feet from where she was…
Apart from the fact that it just feels good, cats need sunlight in order to produce Vitamin D, important for bones and fur.
It is also meant so their owners (really not a good word, since the cat owns the person) can enjoy the view of this beautiful animal, sunning itself, looking smug, not having to work, lazing around… do I need to go on?
I want to be a cat in my next life
I may be mistaken, but I don’t think cats need to synthesize vitamin D. Humans, of course, do. Cats are covered in fur, which effectively blocks the skin from sunlight, so I don’t see how the photochemical reactions could occur in their skins…
I vote for the feelgood hypothesis.
Neurodoc, you are right and thanks for correcting me on this. Apparently I am not the only one who thinks that cats synthesize Vitamin D with the help of sunlight, it’s all over the internet, cat books, etc. But I found this on a site that deals with animal nutrition:
“For most animals, vitamin D is a conditional nutrient in the diet in that it is only when that animal does not receive exposure to sunlight that it becomes an essential nutrient. Dr J.G. Morris recently demonstrated the inability of kittens to synthesize vitamin D when exposed to sunlight or UV light. The explanation is that the 7-dihydrocholesterol (the precursor for vitamin D synthesis) is in very low concentration in cat skin. In the wild state cats, obtain sufficient vitamin D from prey. Although we know now that vitamin D is an essential nutriment in cats, the dietary levels already recommended in commercial foods will fulfill all their requirements.”
It has nothing to do with being covered in fur, though.
To paraphrase Walter Pigeon “They do it because it feels good”
It’s not just the sun, it’s the warmth.
The house I grew up in had state-of-the-art 1950s “radiant” heating. This involved pipes of hot water running beneath the floor. They always broke or clogged up, so certain parts of the floor would be ice-cold and other parts red-hot. The cats would always be found sleeping happily over the hottest part of the floor, which is how we knew where to avoid stepping when barefoot.