it was quite weird at Tsukuba circuit, all the fast guys were out and aiming for their fastest lap in the first or second session, the m speed guys reckon that track loses about a second or so a lap around lmidday on.
but like has been said many variables, i think the tyres they were running wouldnt like running later.
myself personally, i tend to go best in about the middle of day before i start pushing to hard and getting untidy!