रात्रिं कथंचिदेवेमां सौमित्रे वर्तयामहे।
उपावर्तामहे भूमावास्तीर्य स्वयमार्जितैः॥
rātriṁ kathañcid evemāṁ saumitre vartayāmahe
upāvartāmahe bhūmāv āstīrya svayam ārjitaiḥ
rātrim = night; kathañcit eva = somehow or other; imām = this; saumitre = Saumitri; vartayāmahe = we should pass; upāvartāmahe = let us sleep; bhūmau = on the ground; āstīrya = after spreading; svayam ārjitaiḥ = grass and leaves that We have personally collected.
Saumitri, we should pass this night somehow or other.1 Let us sleep on the ground after spreading grass and leaves that We have personally collected.
1 Saumitri means “son of Sumitrā” and refers to Lakṣmaṇa. It is commonly used in Rāmāyaṇa. Similarly, Dāśarathi (“son of Daśaratha”) is also commonly used to refer to Rāmacandra.
“Somehow or other” because it was the first time They are passing a night [without Sumantra and outside Their kingdom]. The night had fallen before They could make [relatively comfortable] arrangements for sleeping.