How long is a long session?

[TheDespot]

When I play live poker, I can basically play an infinitely long amount of time. However, once I switch to online I find myself sort of tailing off and losing focus at around the 2 hour mark. In addition, I only single-table online. I tried multi once and I really didn't like it, as I found it much harder to pick up reads, tendencies, etc. I'm sure most of you on here usually put in longer sessions than this. I want to, but I just sort of daze off after about 2 hours. This only applies to cash games for me; if I go deep in a tourney and it lasts for longer I still play as well as I can (though I do not play tourneys that often). Anybody got a suggestion how I can get more hands in at a time? Or is this even a bad thing?