With more interest in daytime naps as a means to cope with the 'mid-afternoon dip'5, research into the relation between heart rate and sleep has received a new ...
HEART RATE DURING DAYTIME NAPS: DEPENDENCE ON SLEEP STAGE H.M. van Zutvena,b, J.C. Visb and E.J.W. van Somerenb,c a
Radboud University Nijmegen Netherlands Institute for Neuroscience, Slaap&Cognitie groep c Vrije Universiteit medisch centrum, Afd. Klinische Neurofysiologie en Medische Psychologie b
INTRODUCTION Brugada syndrome6 and obstructive sleep apnea3 are two examples of diseases affecting both sleep and heart rate activity. Often an electrocardiogram is used in clinical polysomnography. It is therefore important to understand the relation between sleep and heart rate. In 1973 Aldredge and Welch studied this relation. They found a decreasing mean and variability of heart rate with deeper night-time sleep1. With more interest in daytime naps as a means to cope with the ‘mid-afternoon dip’5, research into the relation between heart rate and sleep has received a new impetus. Heart rate seems to follow a circadian rhythm reaching its highest value during the early afternoon and lowest value during the early morning7. The fact that an afternoon nap would coincide with a peak in heart rate might alter the relationship between sleep and heart rate parameters as compared to nocturnal sleep. The results of Aldredge and Welch1 were therefore compared with data from a recent study that used a daytime nap. Stage 2 was compared with the combined stages 3 and 4 (slow wave sleep, SWS).
METHODS Nine subjects participated (mean age 28, standard deviation 6) in a study on sleep and memory. They spent two afternoons in the laboratory on one of which they slept. During the other afternoon they listened to music and read material of their own choice instead. This afternoon will be disregarded for this study. To increase sleeping pressure the subjects had to postpone bedtime for 2 hours the night before and wake up at their normal time. On the night before the study they were not allowed to drink amounts of alcohol that would, to their own judgment, affect their sleep and performance. During the morning of the study they were not allowed to drink coffee. They had a week to recover from the unusual sleep time between the two experimental days. Duration and quality of the nocturnal sleep was estimated with an actigraph (actiwatch-L), which measures (wrist) movement and light intensity4. All subjects slept adequately the night before the afternoon testing session in the laboratory. The time window in which the subjects tried to sleep in the lab started around 3 p.m. (standard deviation 33 minutes) and lasted 1 hour and 16 minutes (standard deviation 9 minutes). Sleep was classified according to the criteria from Rechtschaffen and Kales8 using Fz, Cz, Pz, and Oz electrode positions from the 10-20 system. The ECG electrodes were placed beneath the right collar bone and on the left false ribs (approximately II). The mean heart rate and its standard deviation were calculated using the R-R interval in every 30-second sleep scored epoch. Next, the average across the total number of sleep epochs were calculated for stage 2 NSWO 18, 2007
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and SWS. Two subjects did not have enough SWS during their daytime nap so they were excluded from the analysis.
RESULTS AND DISCUSSION Table 1. Subjects spent two days in the laboratory. Data from the day they slept are listed below in beats per minute (bpm). Standard deviation and mean heart rate were first computed for each 30 second and then averaged per sleep stage. Sleep stages 3 and 4 were combined to SWS. Subject Laboratory day
