Which is true - sleeping makes you thinner, or staying awake makes you thinner?

I agree with Jazlovestoskate, ….. . ….. but to give a direct answer to your question, you get thinner by consistently burning more calories than you consume. Your body burns around 70 kCal per hour while you are sleeping, around 90kCal per hour while you are awake and sitting down, and around 100 kCal per hour while standing up doing nothing else.
. So it might seem that being awake burns at least 20kCal per hour more than being asleep, thus helping to burn more fat and get thinner PROVIDED you are consistently remaining in “calorific deficit”.
. However 1.0 kg (2.2 lb ) of fat is equivalent to 9000 kCal so if you reduced your sleep from 8.5 hours per day to 7.5 hours per day and sat around awake instead for an extra hour per day, it would take nearly 18 months to burn off that extra 1 kg of fat assuming all other variables remained constant and in both cases you remained in calorific deficit.

However, in practice, you do not normally consume calories while you are asleep (excepting constant drip-feed while in hospital) so the longer you are awake, the more opportunity you have to eat some extra food. . On average, you are statistically likely to consume around 120 kCal (or more) per hour while awake compared to 0 kCal while asleep, so although you may burn at least 20 kCal more while awake, you are also very likely to eat much more than that - thus making it true that in practice people who are dieting to lose weight tend to lose a bit more if they sleep an extra hour instead of being awake (and being more likely to have a snack without exercising more).
.

– Best wishes - Majikthise. .

I have read before in yahoo that when you sleep late, you tend to go fat.

Same! I just read that on Yahoo!

If sleeping did make me lose wieght, I’m pretty sure most girls would go to sleep most of time. And I would use that excuse for not doing anything.