For some time we have had fixed header on our sites. The problem with fixed headers are – they are fixed. If you are not using it, its taking up good real estate.
For people using the space bar to scroll it is even more annoying. Then you have to scroll a bit up, because the spacebar scrolls one page at a time.
The solution is to apply position fixed when the user scrolls up and position static when the user scrolls down. Throw in some CSS transitions to make it less jumpy.
When you scroll down you are reading or looking for something downwards on the page. If you scroll up you might be heading for the menu. At least you are not reading continuously.
We are implementing this on our pages in a few weeks.