Work in 60 to 90 minute time slots. Your brain uses up more glucose than any other bodily activity. Typically you will have spent most of it after 60-90 minutes. (That’s why you feel so burned out after super long meetings.) So take a break: Get up, go for a walk, have a snack, do something completely different to recharge. If you are at a computer and stare into a screen all day, then you need to get up and look away from the screen.
Yes it may take a little longer to complete stuff, but you WILL complete it! Also you don’t want to make mistakes, which can happen if you work straight through and get tired. Your health comes first and good work comes second!