Hundreds of buyers of “Half-Life 2,” the hotly anticipated shooting game released last week, are reporting in developer Valve Softwares user forums that they feel queasy and often develop a bad headache after playing the game for as little as an hour.

From the comments, here is an itneresting take on the problem:
“Most First Person Shooter games have a fov of 90 degrees. The default in Half Life 2 is 75 degrees.

…Go into options and go to keyboard, hit advanced options and enable the developer console “~” then go back into the game and press the tilde “~” key to enter the console. Type these commands:

sv_cheats 1
fov 90″

More here at CNET.

Considering I had the same problem with HL, I fear I can not play HL2 either. 🙁 – Atomm