There is no definitive answer to this; as you outlined yourself in the article and comments this does depend on your energy levels which depend by itself on your cortisol levels. We are different people; so, what works for one person might not work well for another...
As you have been a night owl for some time you have had a shift of your cortisol/energy levels in such way that they are high in the late evening or at night; this makes it so that you can concentrate yourself much better in the late hours while others simply can't think about something else than sleep.
It might or might not be that your cortisol/energy levels are not back to normal, it kind of depends on when you had your switch back to a normal sleep schedule. In the case that your cortisol levels are back to normal, you can learn through-out the day at normal energy levels and spent the rest of the time on something else. Learning at the evening can be tricky, given the lack of cortisol/energy at that point it's much harder to concentrate on your study. Some people decide to go sleep and wake up earlier to get the benefit of the risen cortisol/energy levels in the morning.
The advantage of learning as a night owl is that there are no social interruptions and you are learning under higher energy levels, this however comes at the cost of social activities: You don't sit together with family in the morning, you are tired when you go out on restaurant, and so on. While you still can, you might want to avoid falling into a Delayed Sleep Phase Syndrome; it's nice that you got yourself out of being a night owl, but you might not be able to do that the next time...
It's up to you whether you want to live a normal social life, or be a night owl and exceed in concentration.
So, you have asked:
Do I need to find out when is the appropriate time to study for me, the time when I'm at the height of my brain power and the least tired?
That's really up to you.
Just study when you feel you could study; figuring out when that is, is likely to result in procrastination...
If you want, you can use software like SleepChart (Download near the bottom) to track your sleep schedule, then you can imagine your energy/cortisol levels among the day as well as when it should be best to learn as well as when you can get the best sleep. It simply works by drawing bars of when you sleep:
If you do it for long enough (several months of data to get an exact graph) you can get a graph like this where blue Homeostatic graph expresses the ability to fall asleep and the red Circadian graph expresses how much hours you would sleep. Please note that 0 on the X-axis is the waking time and is not 0 AM.
The bars in the bottom were drawn manually, but are easily derived from the graph as well as your daily activities. As you can see, drinking a coffee 8 hours into this individual's day decreases the necessity to go sleep. The dots are just data points used to form the circadian graph; I believe the homeostatic graph is derived from there using a formula...
You can also see that when the cortisol levels are low you can get a lot more sleep if you were to go sleep at that moment. For this individual, there is a huge difference of 2 hours of sleep if he goes to sleep 13 hours into his/her day rather than 15 hours into his/her day.
The above graph is from someone who sleeps during a Siesta, so the Homeostatic graph might be more like the Circadian graph (not high 8 hours into his/her day) for non-Siesta sleepers.
For DSPS patients it also allows one to see the shift in sleep, more information on this and other uses of the program can be read at SleepChart (Download near the bottom).
