reverse bedtime

yes, there will definitely be changes.

how does it matter .... your body need minimum rest ... it does not matter when you sleep

There's a major effect. We all have a circadian rhythm. We don't choose it and the hours of sleep we get is so much less important to the timing of sleep. Our circadian rhythm is a hormonal cycle, primarily involving cortisol, vitamin D (not actually a vitamin) and melatonin. The vitamin D is what we get from the sun and lets us know that it is daytime and that we should be awake (among other things). The cortisol spikes in the mornings to jump start everything. Melatonin kicks in when it's dark to let us know it's night and that we should power down. Disrupting this really confuses the body and it takes time to readjust, but even after a very long time, the body still wants to have its normal sleeping cycle back. Working a night shift is bad for the health and there has been studies on it, measuring the effects. Since the circadian rhythm is hormonal, throwing this off throws off other hormones as well, so people can become more depressed, anxious, and just more stressed in a general way. It can work the other way around, as well, which is why people who are depressed or anxious may experience insomnia or over-sleeping. Chronic fatigue syndrome isn't completely understood at this point, but has been shown to be tied into hormonal imbalance. If you can keep a healthy sleep cycle, do it the best you can. It really has a huge impact on quality of life.

Do what you are comfortable with.

Some people do have their nights and days muddled up. This is often caused by working night shifts when workers have to eat at strange times of the day and their bodies get used to it.

If you feel more productive to your chosen time then it is just fine. However see to it you still follow the required time of sleep for your age.

I like it. We stay up until the wee hours of the morning. For instance it is 2:44 am here now. We are fixing to get ready for bed. We will sleep until 11 am - maybe a bit longer or maybe we will get up earlier. I find that I get much better rest. I don't toss and turn and lay awake. I go to sleep and sleep hard. It is working for my husband as well. He has spent lots of nights awake all night long. So we are both getting more and better sleep this way. We are both retired so don't need to get up at a specific time.

The body clock would adjust accordingly. It can be tough initially and one has to be very careful about the digestive system.

I used to work nights and sleep during the day and it had no effect on me.