야작하다 is short for 야간작업(을) 하다 or 야간작업하다. 야간작업 = night work (밤일). (야간 = formal word for 'night time', 작업 = work, work procedure) * 야간열차 = night train (train operating at night). * 야간비행 = night flight.

Thanks!

Do night work. I think I'll stay at school tonight and work at night.