Sir John Hawkins, The Life of Samuel Johnson, LL.D.
(London: Printed for J. Buckland et al., 1787), p. 560:
When retired to rest, he indulged himself in the dangerous practice of reading in bed.
Presumably the danger lay in the possibility of fire from a candle after the reader nodded off, but cf. Robert Darnton, The Kiss of Lamourette. Reflections in Cultural History
(New York: W.W. Norton, 1990), pp. 171-172:
In a tract of 1795, J.G. Heinzmann listed the physical consequences of excessive reading: "susceptibility to colds, headaches, weakening of the eyes, heat rashes, gout, arthritis, hemorrhoids, asthma, apoplexy, pulmonary disease, indigestion, blocking of the bowels, nervous disorder, migraines, epilepsy, hypochondria, and melancholy."
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