Naked. I have ever since I got my own room when I was ~10. I always hated sleeping in clothes, but I cannot sleep unclothed in a room with other people. 24, BTW.
I seem to remember having all sorts of PJs and nighties as a kid, but I also remember getting in trouble for not wearing them, so I think I must have worn them under protest.
I’ve mostly slept nude since I had my own room in college. I appreciate a flannel nighty if the room is cold, but PJs while good loungewear shift around and wake me up with their uncomnfortable bunchiness. They end up in a pile next to the bed.
27, female.
By myself – a shirt of some sort, usually an oversize t-shirt, and underwear. I can’t stand having a waistband at night, so I don’t wear pants. If I’m hanging around I have a pair of pajama pants or, if it’s chilly, sweatpants that I wear.
Should I be lucky enough to sleep with a guy again – nothing, almost definitely.
Age 0 - 5ish: pajamas, usually flannel or cotton, with snaps, and sometimes feet. These were not my choice.
Age 5ish - 12: tighty-whities and, if cold enough, a tee-shirt. If really cold, sweatpants. Heck, I’d run around in my BVDs as much as possible.
Age 12 - 18: Slept in the buff, usually. Sweats and/or tee in winter.
Age 18-23 (college): strictly sweats, with or without teeshirt
Age 23-37 (bachelorhood): buff whenever possible, because I could.
Age 37-38 (living in sin): flannel sleep pants and tee shirts, sometimes nude after… ahem… nude activities…
Naked when I sleep alone; nekkid the rest of the time.
I think I stopped wearing pajamas around 7 or 8 but I’m not sure: sometime between then and 12 I was sleeping in t-shirt and shorts.
Then, when I was living alone for the first time, I went all out and started sleeping in just my shorts and undies 
Oops, I forgot…and socks!
Now, if my clothes, especially my socks, get uncomfortable I take them off. But if one sock gets uncomfortable I have to take off the other one: having just one sock on is too annoying for me to get asleep!
Pajamas and nightgowns during the winter, undershirt/tanktop/t-shirt and underpants/short pajama bottoms during the summer. Started following this pattern by about 6 or 7. As far as I can tell from pictures, I practically lived in blanket sleepers from 0 to 2.
Just the pants part of pajamas, nothing underneath.
As a kid: underwear, pajama pants, T-shirt, and socks.
I have no idea when I switched, or whether I went through any transition period. I’m 33.
As a baby/toddler: diapers and whatever else my mom put on me :).
As a kid, until about age 12: Tighty whities and a T-shirt in the warmer months, pajamas in colder months.
As a teen: ditched the PJs and wore t-shirt and underwear. Been doing this ever since.
In the summer or when the weather is warm, I sleep in my underwear or in the buff. When the weather gets cold, I wear pajamas.
I actually had a phase in Middle School and into High School where I would just sleep in my clothes. My reasoning was that it would save X seconds in the morning, and so would be one less thing I would have to do when I got out of bed 
I slept in pajamas until my twenties, then went nude for a few years. Went back to pajamas when my daughter was old enough to toddle and prefer them now.
I slept nude for many years, but recently (since I moved to Wisconsin) discovered the joys of flannelpants and t-shirts in the cooler months.
Also, I have young neighbors (4 & 6) and they don’t need to be seeing Krebnar’s magnificent bits 
(that’s for Mrs. The Magnificent only…) and we tend to forget and leave the shades open quite a bit.
This summer, it was boxers and t-shirt optional.
KTM
PJs until I was about 12 or so, then nada.
Still nada.
I’m 39.
When I was very little, I was a HUGE fan of nighties. As I grew up a little more, I became fond of sleeping in tshirt and shorts. Now, I am 19, I like to sleep in either a tee/tank & panties/thong, or naked, or just panties/thong. It depends on my mood.
Went nude when I hit puberty, around 13-15 or so. I’m Thin but well endowed, and the undies aren’t made with a proper crotch/waist proportion for me to be comfortable. I sleep nude, and go commado everyday. Haven’t worn undies in 10 yrs or so.
Nude or just undies in the summer. In winter, somewhere between nude, undies, undies and a t-shirt, and t-shirt and pyjama bottoms. I guess it hit me that I could sleep nude after I moved to NM for grad school, and didn’t have roommates anymore (2.5 years ago). Prior to that, I’d always wear at least a tank top and boxer shorts, but then Oregon was a little nicer for wearing the extra clothes to bed.
Nude sleeper almost all the time. I’m 23, and have probably slept nude since… oh, jeez, I don’t know… a long time. Since maybe I was 12 or 13.
Am I the only one who can’t sleep if they’re naked? I know I have minor sensory issues since I can’t settle down enough to sleep without a blanket (for the weight, not necessarily warmth. I have very light-weight blankets for the spring and fall) or heavy sheet over me. Sleeping with nothing on just doesn’t work for me. If it’s deathly hot a t-shirt and nothing else will work, but completely naked doesn’t lead to sleep. Most of the time I wear a t-shirt and sweat pants or shorts.
I have an unnatural love for pajama pants that can’t easily be explained so I pretty much always wear them and a t-shirt. If it’s really hot, then nude. I still live with the parents, so the nude sleeping is generally at a minimum. I dunno, I keep thinking they’ll wander in.
-Lil
We were raised with the rule that clothing: pj’s, undies, especially socks, were a big no-no and that it was very unhealthy to keep your body covered with anything but bedding and a either naked or loose pair of shorts while sleeping.
My Mother always said it was very unhealthy to sleep in clothes and that the body needed to breathe at night.
It’s probably just an Old Wive’s Tale - I can’t find a study to cite one way or the other. I still follow the no sleepwear rule and it’s what I’m used to. I may have gone on to perpetuate Mom’s incorrect advice to the next generation - because neither of my kids ever sleep in socks or undies…but they do use fleece PJ’s in the winter