Are you taking Vitamin D supplements? A lack of vitamin D can lead to tiredness sleep problems. Unlike most supplements (where the NHS advice is that most people don't need them) the official NHS advice states that you probably should be taking Vitamin D.
Quote:
"During the autumn and winter, you need to get vitamin D from your diet because the sun isn't strong enough for the body to make vitamin D.
But since it's difficult for people to get enough vitamin D from food alone, everyone . . . should consider taking a daily supplement containing 10 micrograms of vitamin D during the autumn and winter"
Source:
https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
See also here:
https://www.nhs.uk/conditions/insomnia/
If you've got no regular commitments that mean you need to be up at a certain time of day though, you could possibly adopt my own strategy, which is simply to go to bed when I feel tired and get I when I feel like it. That often means going to bed anywhere between 3am and 8am, and then getting up anywhere between lunchtime and teatime. It might seem odd but it works for me!