SUPPLEMENTAL:
Laure's answer is excellent, and describes formal use of this suffix correctly: -ble is used with transitive verbs to express capable or worthy of being VERBed.
It should be noted, however, that in colloquial use—and even more in faux-colloquial writing such as advertising—there is a growing tendency to extend the suffix to intransitive verbs to express the sense usable or suitable for VERBing. Your own example, sleepable, is just this sort of use: it means the hostel in question is usable for sleeping despite the nearby factories. Here's another:
[Franz West] spent a lot of his time sitting, being on his posterior. He made sittable sculptures. He started doing this in the mid-1980s, he started to make these things you’d sit on. link
Creating new -ble words from intransitive verbs is not currently acceptable in formal English, but it should not be regarded as an “error”. It is a natural extension of the sense; and historically, in fact, it represents a return to the original sense of the Latin suffix -a/ibilis, which expressed both ability and fitness. Several fairly common words, which entered English directly from Latin, or from Latin via French, have the suffix in this sense: terrible, horrible, comfortable. I have little doubt that in another generation or two the use with intransitive verbs will again be generally acceptable.
The -able/-ible distinction, by the way, reflects differing Latin stems: Latin verbs ending in -are in the infinitive take -able, others take -ible.
