I don't know much about Metformin, but if you're feeling unnaturally tired for no obvious reason, then in my opinion it's always worth having some general tests eg iron levels, Vit D, coeliac screen, thyroid.
Testing in the night is hard when you're tired. What I do is set an alarm on my phone, put my phone on the floor far enough away from my bed so that I have to get out of bed to stop the alarm, and put my meter beside it. Then the alarm goes off, I roll out of bed, switch it off, test, then it's all done in a minute or two and I just go back to bed and carry on sleeping.