Absolutely. I actually did it last night because I was trying to fall asleep (I was anxious) and it worked. If I could teach people with anxiety only one thing it would be diaphragmatic breathing. It does take a while to get the hang of though. Have a look at a picture of what the diaphragm actually looks like then you can visualise what it's doing as you breathe. When you breathe correctly you stimulate the sympathetic nervous system, which is not working properly when you're anxious because the parasympathetic is in overdrive. It basically thinks you will be attacked by a bear if you fall asleep. So what I do is focus on my breathing and calming the nervous system. It's not some magic bullet and I've had to practice, but I've found it really does work.