Hello Kristinaolga, and welcome to Piano Society!
I think it's purely individual. As long as your brain is awake enough to concentrate and your body is strong enough to function at the piano properly, I don't think it matters what time of day. I used to practice any time throughout the day. Now I practice mostly at night. Then again, now that I think about this, it may be better to get your 'serious' practicing earlier in the day when you have not yet worked all day or done whatever it is you do all day long. I'm finding that my concentration level at night is not as long as it used to be. I practice for an hour and then I feel spent. So I'm changing my answer - it's better to practice earlier in the day.