Before our daughters went to university, their dad would either lie in bed not sleeping, or sit on the sofa wide awake until what ever ungodly hour they arrived home. He repeated this when they came home for holidays but when they were away at uni, he slept like a baby.
I did worry when one of them was off travelling for a year but Skype, email and Facebook meant we never had more than a few days without knowing all was well. I remember my cousin going off to Goa, overland, in 1968 - nothing heard for months until a postcard arrived to tell his parents he was still alive.
I honestly worry more if mine are driving down the M6 or M1, as they regularly do, than I do if they're flying somewhere.