Arousal threshold determination in 1862: Kohlschütter's measurements on the firmness of sleep

Abstract

A multitude of studies used external sensory stimuli to experimentally induce electroencephalographic, vegetative or behavioral reactions in the sleeping subject, either to gather information on the nature of sleep or to induce sleep fragmentation. But Ernst Otto Heinrich Kohlschütter was the first to systematically investigate the change of awakening thresholds across the night, generating a sleep depth curve for his 1862 dissertation "Measurements on the Firmness of Sleep" ("Messungen der Festigkeit des Schlafes"). For the time, his concepts of sleep and the design of his experiments were impressive. A re-analysis of his data with modern regression techniques and a comparison with a polysomnographic laboratory study on the effects of traffic noise on sleep shows that he correctly captured the typical evolution of awakening thresholds across the night, with only 74 measurements in a single subject. Nevertheless, Kohlschütter's analyses were hypothesis driven, as he discarded almost 50% of the data points to derive what he called an "idealized curve of sleep". This does not belittle his achievement of being the first to systematically investigate arousal thresholds across the night without the help of electrophysiologic measurement techniques.