@overtheedge - the breathing exercises
@mattie recommends are good.
Also, simply placing your hands on your abdomen (over the solar plexus point I believe) while laying down is calming. You just lay there and keep your hands in place. If you combine this with box breathing (similar to
@mattie's suggestion) it would be very calming.
I would combine doing this with your hands and actually focusing then on breathing into your hands, into your stomach/solar plexus and chest as deeply and slowly as you can tolerate, or whatever feels comfortable. It can be an interesting exercise actually to focus on breathing into different parts of the body, and focusing your attention in different places. This can also help to get out of over thinking as you are literally focusing your brain and attention elsewhere, it is essentially a form of brain training/neuroplasticity as well, if you keep doing it regularly you are activating different brain states.
All that said I have done different breath work for over 8 or more years now, since trying to get into Wim Hof at the time, and slowly realised it is good to simply try to work out what works for you as well, work out what feels good and what is helping, experiment. For me it can be doing simple buteyko and breathing into my stomach just trying to get into a more relaxed state.