I'll echo the thoughts of the others. With the exception of 1 rotation this year, I am in house for all nights on call. For the home call rotation, there was time to bruch my hair but frankly I would not have stopped to do much more (ie, wash, curl, etc.) for 2 reasons: 1) someone is WAITING for you and 2) the sooner you get to the patient, the sooner you finish and the more sleep you can get before having to start the next day.
For in-house call, I generally do nothing to my hair if I get called for a trauma or have to go see a patient on the floor/SICU - maybe just put it up in clip. ALL of us look bad, so there really isn't much to worry about. Believe me, I'm a very vain woman but SOME THINGS you just have to give up - I'll even start morning rounds without showering and putting make-up on (because if you've been up all night, or have to start writing notes at midnight to get them all done before 5 am, there just isn't time).
Hope this helps.