General anaesthesia for surgery can influence circulating melatonin during daylight hours

Both melatonin and anaesthetics have been shown to affect sleep and behaviour. The effect of general anaesthesia on circulatory melatonin has not been reported, but anaesthetic-related alterations in hormone profiles are known. We hypothesize that differences in recovery from anaesthesia may be associated with differences in circulatory melatonin levels because of melatonin's sedative effect in humans.

The influences of general anaesthesia and surgery on circulating melatonin, prolactin, and cortisol concentration were investigated in 32 female patients scheduled for elective gynaecological surgery to study differences in hormone profiles and responses during anaesthesia and the recovery period. Patients were randomly assigned to one of two groups. General anaesthesia was induced with either thiopentone/fentanyl (Group 1: n = 16) or propofol/fentanyl (Group 2: n = 16). Maintenance of anaesthesia was achieved with either isoflurane (0.8-1.0 vol%)/fentanyl (Group 1) or propofol (6 mg.kg-1.h-1)/fentanyl (Group 2) with a N2O/O2 flow ratio of 2:1 in both groups. During anaesthesia, patients' eyes were carefully taped shut to prevent light effects. Blood samples were taken before and after premedication, immediately before induction of anaesthesia, every 15 min during anaesthesia, and hourly in the recovery room for 8 h. The control group consisted of 6 healthy women who were not subjected to surgery, but who were in a similar environment, including light conditions, as the study groups.

Isoflurane and propofol anaesthesia as well as darkness elicited elevated plasma melatonin levels that persisted in the recovery period in patients anaesthetized with isoflurane, but gradually decreased during the recovery of patients anaesthetized with propofol. Circulating prolactin and cortisol values were also elevated during anaesthesia and had similar decreases during the recovery period.

Higher plasma levels of melatonin during the recovery period following isoflurane anaesthesia may, in part, explain increased sedation in these patients compared with patients who received propofol anaesthesia. However, the relationship between recovery from anaesthesia and plasma melatonin levels may not be simple and straightforward.