Rest refers to rules set by the IRS that describe a sufficient amount of downtime for a transportation employee subject to DOT rules.  This includes aviation industry employees such as a flight crew that is deducting travel expenses. The rest requirement, along with the requirement that the flight crew must be away from their tax home, allows professional flight crews to deduct traveling expenses. Merely napping while at an outstation does NOT satisfy the rest requirement described in IRS Publication 463.  This is why a flight crew cannot write off travel expenses for day-trips. A pilot or flight attendant's relief from duty should be long enough to get necessary sleep or rest.

 

The following is a paraphrased example from IRS Publication 463:

A railroad conductor leaves his tax home on a round-trip run between two cities.  The conductor returns back to his tax home sixteen (16) hours later. During the trip, the conductor has six (6) hours off at the turn-around city where he eats meal and rents a hotel room to get sleep before starting the return trip.

In this example, the 6 hours rest that the conductor received is enough to constitute a necessary rest period to justify deducting travel expenses, including meals and incidental expenses (M&IE) while away from his tax home.  Note that this does not necessarily have to correspond to the rest rules that the FAA regulations describe for airline pilots.

 

The definition of what constitutes a rest period can be a little gray, but generally for pilots and flight attendants to deduct travel expenses (including the per diem deduction), they need to be on a layover.  If a day trip has enough of a break in the middle that it constitutes a hotel or some other rest location, then the pilot or flight attendant is probably eligible to write off travel expenses because they had enough of a rest period to meet the IRS definition of rest.