I would suggest keeping a daily diary of what you eat, your energy level, your mood, how much you slept and your sleep quality (insomnia, hypersomnia), and any

I would suggest keeping a daily diary of what you eat, your energy level, your mood, how much you slept and your sleep quality (insomnia, hypersomnia), and any other symptoms or potential causes you'd like to track. A Google spreadsheet would probably work well. If you are disciplined and track good data, if after a year you haven't figured anything out yet, you've got a boatload of data to analyze and may be able to unlock the mystery yourself.