This is the 11113th most frequent Turkish word.
durumdayken
"While in the situation/condition."
The word 'durumdayken' is used here to describe a condition or state ('yorgun' - tired) during which the action occurs (studying).
Yorgun bir durumdayken ders çalışmak zordur.
When in a tired state, studying is hard.
Here, 'durumdayken' indicates a condition ('meşgul' - busy) present at the moment of difficulty in meeting friends.
Meşgul bir durumdayken arkadaşlarınla buluşmak kolay değil.
Being in a busy state, it is not easy to meet with friends.
In this sentence, 'durumdayken' connects the state ('sakin' - calm) of the subject with the rationality of making decisions.