There is a vast literature dealing with various aspects of the circadian clock. This review provides examples of the multifaceted nature, of namely redox, oxidative stress, melatonin, antioxidants, nitric oxide, hypoxia, anesthetics, cortisol and cocaine. The redox and oxidative stress features reflect the involvement of electron transfer and radical species, which can exert either beneficial or deleterious effects. The harmful aspects can be countered by antioxidants. Melatonin is a brain hormone that is known to play a widespread role in the biochemistry of the circadian clock. Nitric oxide is widespread in the human body and plays a variety of roles. Hypoxia has been broadly discussed in relation to circadian rhythm, e.g., in high altitude flight. The anesthetic topic completes the list.