I have also learnt that I am the same when it comes to my writing. I need a stretch. Thank Dickens!! Some writer somewhere, sometime, came up with a writing stretch.
I know illustrators who do warm ups. Nothing that will make it into a book or their portfolio, but some thing to loosen up their drawing muscles and creative juices.
As a writer I need the same thing. Jumping into hard core writing right off is like trying to comprehend the Theory of Relativity while I'm in a coma. By the way, I don't completely get much of Einstein's work when I am wide awake.
I use a book purchased at my local book store, pictured above. There are many excercises suggested on many websites.
Some are short, such as "Create an Ode to an Onion"; or longer ones such as "Fix the Plot of the Worse Movie You have Seen."
I have found these helpful even when it isn't a writing day. It keeps me in the groove and makes it easier to get started on a scheduled writing day. It is useful when waiting at a doctor's office or on public transit. I prefer to use pen and paper but the smartphone can come in very handy too.
So do that morning stretch and drink the gallon of coffee. Then find a little story to tell. Who knows, it may be the beginning of your next best seller.