It means sleeping is something you like in general (i.e. an activity you enjoy).
If you want to sleep right now you can say Я хочу спать.
We do not distinguish between "like to do smth" vs. "love to do smth" the way English does. Мне нравится спать and Я люблю спать are roughly the same in strength (the latter is probably a more common wording).
If you need to express a stronger feeling of liking something, you can add "very"(e.g., очень люблю + infinitive) or use some other verb (e.g., обожаю "adore" + infinitive).
It is conversion, a pretty common thing. The peculiar thing is, English easily converts words to a different class without any overt marking.
For example, in Russian silver as material is серебро and silver as adjective is серебряный. The verb ("to cover, coat with silver") is серебрить. In English it is all just "silver.