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This paper describes an unconstrained pneumatic method of estimating the δ-wave activity of the brain from the heartbeat signal. Based on experiments showing that transinformation of the heartbeat signal corrupted by body movement was correlated with the δ-wave activity, we developed a method of estimating the percentage of the δ-wave included in the EEG from the transinformation. The comb filtering technique was used to obtain accurate transinformation. We applied the proposed method to young normal subjects to evaluate the method. As a result, the correlation between the δ -wave included in the EEG and the transinformation was 0.727 and the average error of the estimates of δ-wave percentage was 14.9%. The δ-wave activity and heartbeat activity were shown to be quantitatively related. This suggests that sleep depth can be estimated from the δ -wave percentage estimated by unconstrained measurement of the heartbeat signal of young normal subjects.