: Is it important "when" to sleep or only "how much" is important? 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

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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:

Remember that cortisol levels normally
peak in the early morning (about 6:00
a.m. to 8:00 a.m.) as a way to get a
person moving and prepare her to face
the challenges of the day. Between
8:00 a.m. and 11:00 a.m., cortisol
levels begin to drop, and they
continue to gradually decline
throughout the day, typically causing
us to feel a decrease in energy levels
and ability to concentrate sometime
around 3:00 p.m. to 4:00 p.m. (the
"afternoon slump"). This dip in energy
levels is the body's way of saying,
"The day is almost over; better get
ready for sleep." Unfortunately,
instead of getting ready for sleep,
our modern lifestyles cause most of us
to search for a way to boost our
energy levels in the evenings so that
we can get through soccer practices,
piano recitals, business dinners, and
time with our families. Our body clock
really wants us to eat our last meal
of the day around 5:00 p.m. and to be
asleep by 8:00 p.m., but our
wristwatch has us awake late into the
night. The major problem with our
modern "late to bed, early to rise"
lifestyles is that our cortisol levels
never have enough time to fully
dissipate (which is supposed to happen
overnight), so our bodies never have a
chance to fully recover and repair
themselves from the detrimental
effects of chronic stress.

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:

Darkness while you sleep
Light while you're awake
Meal times
Stressors, both dietary and lifestyle

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.

Researchers from the University of
Helsinki, in Finland, have shown that
cortisol causes fat accumulation in
specific sites most likely to be
associated with insulin resistance
(prediabetes). These sites are
abdominal fat tissue and the liver—and
fat accumulates in these areas because
of the activity of HSD. Higher
activity of HSD means a higher rate of
fat storage and a faster accumulation
of fat stores. In abdominal areas, a
high HSD activity leads to a rounder
waistline, while in the liver it means
a higher risk for diabetes.

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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:

Extra light, sending the message to your body that it's time to be up
Rush of energy after your body is done processing carbs 12 hours earlier (or 6-7 hours after alcohol)
Extra noise, sending the message that you need to be aware of what's going on around you.

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.