An affix comes after a word in the Nominative 1st-Case in construction denoting time in the sense of 'this is his habit or endurance'.,
The word कालात् is understood here also. तद् = that : सोढम् = जितम् or अभ्यस्तं \endured or habituated\. Thus निशा सहचरितमध्ययनं = निशा \reading by night\. तत् सोढमस्य छात्रस्य = नैशः or नैशिकः 'a student who is habituated or enured to reading by night'. So प्रादोषिकः or प्रादोषः ॥,