My players are going to be traveling overland from NYC to Seattle and the rigger is going to be "on watch" over the convoy while they are on the move. Thing is after a while he is going to get tired/lose focus should I just treat him as distracted (-2DP) or is there a better (proper) way to do it?

p. 256, SR4a
I believe Sleep Deprivation is the thing you're looking for.
It's hidden well, as a 'toxin', but it should serve.
Long Haul is rigger's best friend, though.

I'd suggest that they hire another rigger he could take turns with.

Overland from NYC to Seattle?

He should get plenty of sleep waiting at the border crossings for their turn in the queue.

Yup. Take longhaul. 2 doses will keep you up (up, up, up) for nearly a week. Long enough to go from NYC to Seattle.

If you go with the stimulant route, make sure you have a plan to sleep it off afterwards under medical care, because when you crash from two doses of Long Haul, you are going to be down for the count, to the tune of 10S unresisted, and you'll stay down for for an average of 28 hours (with a range of 8-48 hours.) So don't have anything you urgently have to do the moment you get to Seattle.

Also, taking a second dose of Long Haul keeps you up for 1d6÷2 days. That could be as few as twelve hours, or as many as 72, with (if your GM is generous) half-day intervals. (If your GM is not generous, it's 0-72, in whole day intervals only, meaning the second dose could be completely ineffective.) Have a plan for what happens if you suddenly and unexpectedly fall asleep at the wheel for 1/3rd of a day to as long as two days, in case the GM rolls secretly and you get 12 hours or 0 hours out of the second dose.

I would also imagine having a sleep regulator would help a lot as an alternative to drugs.You'd only need a good nap rather than a full night's sleep.

I wasn't so much talking about being tired due lack of sleep but tired from focusing on a dozen data streams for 10-12 hrs.

This sounds a lot like a job for an Agent or a Sprite to help out with.

Sensor+Clearsight is pretty easy to get dice pool bonuses from, with, say, Vision Enh 3.

Hm, obviously 24 hours before effects set in would be a bit long for that. How about simply diving a cumulative -1 every n hours?

Maybe some sort of endurance or willpower test? Get enough hits and there are no penalties for the period on watch. Glitch and you drop a couple of balls and could miss something outright.

There's a joke in there somewhere about playing with balls and driving...

Well, if you don't see it maybe you should quit while you're ahead.

..indeed , there is: by Robin Williams: Weapons of Self destruction