Avicenna in the Canon of Medicine said,
“Exercise and heating agents set in motion the sanguineous humour, the bilious humour, and even the atrabilious humour (which is strengthened thereby).
“Repose sets the serous humour in motion and strengthens it. Repose also strengthens some kinds of atrabilious humour.
“Even imagination, emotional states and other agents cause, the humours to move. Thus, if one were to gaze intently at something red, one would cause the sanguineous humour to move. That is why one must not let a person suffering from nose-bleeding see things of a brilliant red colour.”