Tiredness Daily: Any Advise?

On my fourth pregnancy and holding a full-time job. This is the first time I'm feeling tired almost daily and struggling to focus at work. I've tried getting enough sleep (at least 8 hours) not overworking myself etc and taking care of my meals. I don't wake up tired but it usually starts between 9 am to 12 pm daily. I'm on prenatal vitamins too. Anyone with the same issue?