Source:http://linkedlifedata.com/resource/umls/id/C1154967
GO: Any process that results in a change in state or activity of an organism (in terms of movement, secretion, enzyme production, gene expression, etc.) as a result of detection of, or exposure to, a period of light or dark of a given length, measured relative to a particular duration known as the 'critical day length'. The critical day length varies between species. [GOC:jid, GOC:pj, ISBN:0582015952, ISBN:0697037754, ISBN:0709408862]