When I last worked at MegaCorp, I would be in the office by 6.30 in the morning and leave by about 3.30 in the afternoon. This was non negotiable and part of the agreement I made with the company. I could work productively for about three hours before the day's meetings and "manager work" started and the rest of the day usually went downhill very fast. I had some dim sense of "preserving my sanity"as a reason for doing this and it worked very well in this regard, but I could never explain clearly why this worked so well.
Now I know. Paul Graham writes an excellent essay on the different notions of scheduling for managers and "makers". A must read for both managers and programmers.
 At MegaCorp "work" had strange meanings - it was mostly measured in the degree you sucked up to your superiors (I am not being condescending here. I watched several masters of this art who would apply a frightening degree of focus on "managing upwards". Very educational but very scary), and how many meetings you attended. By God, did we have meetings. One of the reasons I left (not the most important one!) was that we started to have meetings about meetings and I came out of these dispirited and demoralized and completely drained.