Sennosides is a mixture of two naturally occurring plant glycosides (sennosides A and B). It works in the large intestine, acting directly on the submucosal plexus and the deeper myenteric plexus to stimulate propulsive waves. The typical onset of action for sennosides is 6―12 hours. Ideally, it should be taken at bedtime, with an expectation of results after breakfast the following morning.

Reference

Twycross R, Wilcock A, Dean M. Canadian Palliative Care Formulary. 1st Canadian ed. Nottingham, UK: Palliativedrugs.com; 2010.