Is sleeping after 12am good or bad?

There's nothing to deal this biologically.:D. The duration to sleep and to stay awake varies based on a person and his profession :)

It all depends on 2 things:

1) what you did during that day's morning?

If you remained to while away your time just by sleeping and didn't to do anything back breaking. You would remain awake even after the clock has ticked past the mid night.

2) what are you up to do the next day's morning?

If you are anticipating something the very next day to happen or you feel the next day's going to be joyful(especially weekends). You are less likely to get a sleep.

We humans are animals. We are nothing different. Don't schedule things to yourself. Do whatever your body wants you to do :D
IT MATTERS. But how does?
Lets see the hormonal aspects of sleep.

CORTISOL is a steroid and one of the primary stress hormones. Normal cortisol levels tend to follow a 24-hour circadian rhythm. The lowest level being at night during sleep that gradually increase to when you need to wake up and get moving. The high cortisol levels present in early morning rapidly drop off and then continue to decline for the remainder of the day. Thus we need to wake up at early morning for high cortisol level.

MELATONIN is a hormone made by the pineal gland, a small gland in the brain. Melatonin helps control your sleep.
Your body has its own internal clock that controls your natural cycle of sleeping and waking hours. In part, your body clock controls how much melatonin your body makes. Normally, melatonin levels begin to rise in the mid- to late evening, remain high for most of the night, and then drop in the early morning hours. Thus we need to sleep at around 10:00 pm with high melatonin level.
Doesn't matter.
Time changes and hence the way of living life is too.

As far as you are getting 8-10 hrs of rest a day you are good to go.
Have a continuous sleep for 6 hours, Thats it.. No matter whats the time, There are people who work in night shifts , They never sleep at night.
it depends upon 2 things
  1. what are u doing till 12? wasting time or making life better??
  2. are u having minimum 7 to 8  hour sleep??

Everyday you should sleep at least 6 to 8 hours to make better nervous system and build a healthy life. Apart from this you can take 15 to 90 minutes naps during daytime to improve brain functions and increase your focus and creativity.

Do university teachers find public speaking outside of the classroom easy?

I do.I've been speaking to a crowd since I was young, though, and never really had any nerves about it.

What's it like to have your car stolen?

It becomes an horrible assault on the psyche, particularly when one's own so-called well-respected American insurance company did it.I still have the pink slip, yet they spirited the car away from a collision center, their intentions were to sell the car at auction