This is certainly true, although many of us have to deal with problems and projects large enough and time-consuming enough that if they prevented us from sleeping properly, we’d never sleep properly. It takes practice, essentially. Experimenting with various approaches until one finds methods that work.

For the particular issue you described, one approach is to journal before bedtime. Write out what is worrying you about the project, its deadline, etc. Put on paper the thoughts that are swirling through your head. Once captured in that manner, they may leave your brain alone for a few hours. It also helps to be organized, so you can reassure yourself that you have a plan.

Then there is the dreaded 3 or 4 am wake up, where all the thoughts can come rushing back in, preventing sleep for the next hour or perhaps even for the rest of the night. I have found that body scan meditation is helpful at these times.

I am not always successful, and these methods won’t work for everyone. The key is, like I said before, experimenting to find methods that work, and practicing the ones that show promise.