I heard in the Rebbe’s name: A Jew’s main devotion in the winter is to wake up at midnight [and say Tikkun Chatzot, the midnight service (cf. Magen Avraham 1:4)]. In the summer, when the nights are very short [and so awakening at midnight is not practiced in the Diaspora³⁷⁰ I, 149; Likutey Halakhot, Rosh HaShanah 3:7; She'arit Yisrael 125.], one should still be careful to wake up early, at the crack of dawn.

footnotes: ³⁷⁰ See Likutey Moharan