On Sleep (Greek: Περὶ ὕπνου καὶ ἐγρηγόρσεως; Latin: De somno et vigilia) is a work by Aristotle, one of the short treatises on nature or Parva Naturalia.
Free online texts
Internet Archive: The Works of Aristotle, Vol III, translated under the editorship of W.D. Ross (1910). Parva Naturalia translated by J.I. Beare and G.R.T. Ross. Multiple formats.
Internet Archive: Aristotle's Psychology: A Treatise on the Principle of Life (De Anima and Parva Naturalia), translated by W.A. Hammond (1902). Multiple formats.
Mikrosapoplous.gr: ΠΕΡΙ ΥΠΝΟΥ ΚΑΙ ΕΓΡΗΓΟΡΣΕΩΣ. Greek text. HTML format.
University of Adelaide (Internet Archive): On Sleep and Sleeplessness, translated by J.I. Beare. Multiple formats.
Wikisource: On Sleep and Sleeplessness, translated by J.I. Beare. Multiple formats.
History of Philosophy without any gaps: Classified Information: Aristotle's Biology - podcast by philosopher Peter Adamson.
Librivox: Parva Naturalia - public domain audiobook.
Stanford Encyclopedia of Philosophy: Aristotle's Biology, by James Lennox.
Wikipedia: On Sleep.
The Great Conversation: Further reading at Tom's Learning Notes
Aristotle: On the Soul (De Anima).
Ancient Greek resources: Learn to read Greek classics in the original.