The best time of day for language learning depends on two factors. What you are trying to accomplish, and what time of day you find that you have the most energy. The task is really to simply decide on your immediate goal, and match it with a block of time in your day when you are most productive.
Let’s select a simple goal for an example. I want to understand how I can order in a restaurant in English. Once you have this goal (something I often refer to as an “end” or “objective” because of my military training) you can sketch out some sort of plan of action. I am going to do it during the time when I most alert and attentive and when language partners are available to roleplay with me.
You have to observe your behaviors and energy levels and record the results on paper or on your computer. This really helps you key in on what time of day is best.
If you are memorizing a certain number of words, perhaps 500 in a month, you don’t always need to be sharp as a tack. Sometimes shallow tasks such as using a certain number of Anki cards you can do with low energy reserves and willpower. Sometimes doing these things with low energy can actually increase your confidence as you add experience bit by bit, day by day.
Now it is time to address the question of “time.” What time of day do you feel most alert, rested and are less likely to be distracted? If you did keep a journal even for three days or so, you are going to get into the ballpark with a reasonably good guess. Select one time and go for it. I personally know that I would love to be an early morning person. I watch all sorts of videos telling me how important it is to literally get started at 4 am because it would give me an advantage over the competition. However, I am most lucid and clear between 3pm and 7pm. Hard workouts also do not charge me up or give me energy. It is actually the complete opposite, where I often feel drained. Needless to say, I do not exercise right before my language study times.
Attempt to work for an interrupted block of time. Even as much as four hours with regular breaks. If you do this every day, you are bound to see excellent progress.