Journal of Human Hypertension (1999) 13 , 449–453  1999 Stockton Press. All rights reserved 0950-9240/99 $12.00 http:/ / www.stockton-press.co.uk/ jhh ORIGINAL ARTICLE Comparison of arbitrary definitions of circadian time periods with those determined by wrist actigraphy in analysis of ABPM data MAH Eissa, RJ Yetman, T Poffenbarger and RJ Portman Hermann Center for Chronobiology & Chronotherapeutics, 6410 Fannin, Suite 833, Houston, TX 77030– 5201, USA Determining blood pressure (BP) values at different daily time periods is a well recognised measure to assess the risk of end-organ damage. However, the use of various definitions of these periods, eg, day vs night, sleep vs wake or arbitrary definitions, makes clinical decisions based on available data difficult. In the present study, we compared BP loads in actual sleep– wake periods to default day–night definition provided by the ambulatory BP monitoring (ABPM) software (day 06.00–22.00; night 22.00–06.00) as well as to an arbitrary definition of sleep–wake periods in children published in Journal of Pediatrics (Soergel et al, 1997) (awake 08.00–20:00 and sleep 00.00–06.00). We used an acti- graph, an accelerometer, to define the actual sleep– wake periods in 46 patients with essential hypertension who are on various treatment regimens. BP data were Keywords: ABPM; actigraphy; circadian rhythms Introduction Previous research has indicated that circadian blood pressure (BP) variability may be a significant deter- minant of hypertensive end-organ damage. 1–4 A noc- turnal decrease (dip) in BP occurs in normal people and uncomplicated essential hypertension; lack of such a dip has been associated with secondary hypertension and hypertensive end-organ damage. 4–6 Unfortunately these studies varied in the method of sleep determination (actigraphy, patient diary or arbitrary fixed time periods). Since sleep has been identified as an important component of circadian BP decline during nocturnal hours, 7 utilising a fixed day–night time definition of sleep, which may not reflect the actual patient’s wake–sleep, may lead to patients receiving an inappropriate classification of hypertension. Also, an inconsistent definition of the sleep period makes comparison of data between studies difficult. The purpose of the study was to determine the Correspondence: Dr Mona AH Eissa, University of Texas- Hous- ton, Medical School, Division of Pediatric Nephrology and Hyper- tension, 6431 Fannin Street, Houston, TX 77030, USA Received 4 March 1999; accepted 5 March 1999 obtained by using Spacelabs 90207 monitors for a full 24 hours. There were significant differences between actual sleep–wake and default definition for BP load. No similar findings were noted when arbitrary definition was used. The proportion of hypertensives was not sig- nificantly different when default and arbitrary definitions were used. Classification of dippers and non-dippers is greatly affected by the definition of sleep interval using the default method. Although some of the misclassi- fications were not statistically significant, their clinical importance must be considered. Determination of sleep and wake periods for analysis of ABPM data should be based on careful determination of actual periods. Using other definitions may not provide complete information or accommodate for individual variation. effect of various definitions of the sleep period in the determination of hypertension among patients undergoing ambulatory BP monitoring (ABPM). Specifically, the number of patients classified as having hypertension during an ambulatory monitor- ing period were compared when the definition of sleep was variably defined by actigraphy, by an arbi- trary definition, or the default setting of the ambulat- ory equipment. Materials and methods Study population Study subjects were 46 consecutive patients with a diagnosis of essential hypertension referred to the Hermann Hospital Center for Chronobiology and Chronotherapeutics, Texas, USA. These patients were being managed with varying regimens of anti- hypertensive medications. The mean age of the group was 42.9 ± 21.8 years (range 7–90). Thirty- eight percent of the patients were males; 58% were Caucasians, 28% African-Americans, and 14% rep- resented a variety of ethnicities.