Altered circadian rhythmicity in patients in the ICU.

BACKGROUND: Patients in the ICU are thought to have abnormal circadian rhythms, but quantitative data are lacking. METHODS: To investigate circadian rhythms in the ICU, we studied core body temperatures over a 48-h period in 21 patients (59 ± 11 years of age; eight men and 13 women). RESULTS: The circadian phase position for 17 of the 21 patients fell outside the published range associated with morningness/eveningness, which determines the normative range for variability among healthy normal subjects. In 10 patients, the circadian phase position fell earlier than the normative range; in seven patients, the circadian phase position fell later than the normative range. The mean ± SD of circadian displacement in either direction (advance or delay) was 4.44 ± 3.54 h. There was no significant day-to-day variation of the 24-h temperature profile within each patient. Stepwise linear regression was performed to determine if age, sex, APACHE (Acute Physiology and Chronic Health Evaluation) III score, or day in the ICU could predict the patient-specific magnitude of circadian displacement. The APACHE III score was found to be significantly predictive of circadian displacement. CONCLUSIONS: The findings indicate that circadian rhythms are present but altered in patients in the ICU, with the degree of circadian abnormality correlating with severity of illness.