Complex sentence: He found that absence of a light dark cycle does not prevent humans from entrainment; on the contrary, light is a weak Zeitgeber for humans, as people kept on a 24 hour light dark cycle tended to free run with periods greater than 24 hours.
Split into simpler sentences: He found that absence of a light-dark cycle does not prevent humans from entrainment. On the contrary, light is a weak Zeitgeber for humans, because people kept on a 24-hour light-dark cycle tended to free-run with periods greater than 24 hours.