The first nightshift you work is always the hardest because you feel so sleepy.
Your first night back on nightshifts is always the toughest because you get so tired.
この場合、次のような言い方ができます。
ーThe first nightshift you work is always the hardest because you feel so sleepy.
「夜勤初日はすごく眠くていつも辛い」
The first nightshift you work is always the hardest で「夜勤初日は辛い」と言えます。
ーYour first night back on nightshifts is always the toughest because you get so tired.
「夜勤初日はすごく疲れるのでいつも辛い」Your first night back on nightshifts is always the toughest で「夜勤初日はいつも辛い」とも言えます。
ご参考まで！
カナダ 
