I usually sleep in the morning till noon, is it really important to sleep at nights rather than morning or other times in a day?
Are there any documented studies about when to sleep?
According to this answer, does it affect weight loss process?
It's far better to attempt to sleep on a natural cycle with the sun, in order to keep your circadian rhythm (and the hormonal cycles behind it) in "tune". From Shawn Talbot's excellent Cortisol Connection:
Here's the full section on cortisol and sleep/insomnia.
In order to really be healthy on a delayed sleep schedule, you'd need to adjust and properly time a lot of factors, including:
Regarding weight, a different chapter examines a fat storage enzyme HSD and its connection to cortisol. It's not quite so simple as saying that more cortisol directly causes more HSD, but there is a more complex relationship. HSD in your cells amplifies their exposure to cortisol and other stress hormones, which results in more weight gain.
I think the most important part is quality of sleep. In other words you want a deep REM sleep and you want your total stleep to be 8 hours a day, ideally. The things that can ruin the quality of sleep for you are:
The reason night time works for most people is that it is both quieter (because everyone else is sleeping) and there is no sunlight waking you up.
If you have a night job, then blackout curtains can be a very important investment. All you have to do after that is deal with the noise. Either invest in a white noise machine or fountain to drown out distracting noise or sound proof your room (very expensive).
As to documented studies, you can try this one.