The affix añ already ordained -- prāgdīvyato'ṇ [[4.1.83]], comes after the word sthaṇḍila being in the Locative -7th case in construction in the sense of 'a sleeper thereon' and the whole word so formed denotes 'a person performing a vow'.,
Thus स्थण्डिले शयितुं व्रतमस्य = स्थाण्डिलः 'a person who has taken the vow to sleep on the bare ground' i. e. an ascetic or a Brahmachari.
Why do we say 'vow'? Observe स्थण्डिले शेते ब्रह्मदत्तः 'Brahmadatta sleeps on the bare ground', not as a matter of vow, but of necessity or pleasure. The word व्रत means a 'vow or an observance ordained by religious codes'.,