Anyone know what might cause the clutch to feel quite heavy after driving on the motorway for a while?

Strange that it seems to feel heavy on the first gear change if I have driven at motorway speed for a while and then feels light again??

Any suggestions?

It's not slipping or causing any problems at all! I just wondered really!

Its an odd one, this. The last couple of cars (Vectra 3.2 GSI and Octavia II) have done this, and I can only assume that it is something to do with the hydraulic side of things (servo?) losing pressure (actually vacuum) after not having been used for a while.