It may seem silly, but it is likely that you will be better at playing free poker at certain times of the day. We all have times in the day when we find that our brain functions better and poker takes a lot of brain work.
Some people are morning people, they can get up early and get straight on with work finding that as the day goes on the more tired they get and the worse they perform. Other people find that they take a long time to wake up in the morning and they therefore do not really find their brain wakes up that early. They will find that they are much better in the evenings.
If you are not sure then you could experiment and play at different times of the day and see when you feel you are playing better. It is a bit difficult to measure as how much you win is not necessarily an indication of how well you are laying as that also depends on how well the people you are playing with are doing as well and of course there is also a certain amount of luck involved.
You will be better off consciously thinking about your concentration levels and when you think those seem to be better. It is worth remembering that starting early in the morning and then playing all day is not a good way to measure because you will get tired. It is better to do an hour a day at different times and then you will get a better idea of when you are concentrating the best.
You may need to experiment over quite a long time period, but it is likely that you will soon begin to see a pattern emerging. It may end up being a bit annoying in that perhaps the best time for you to play is the least convenient, but at least you will know and you may be able to fit your games around that a bit.