- Circadian interleukin-6 secretion and quantity and depth of sleep. Vgontzas, A.N., Papanicolaou, D.A., Bixler, E.O., Lotsikas, A., Zachman, K., Kales, A., Prolo, P., Wong, M.L., Licinio, J., Gold, P.W., Hermida, R.C., Mastorakos, G., Chrousos, G.P. J. Clin. Endocrinol. Metab. (1999)