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Lyman-alpha photons from the first radiating sources in the Universe play apivotal role in 21-cm radio detections of Cosmic Dawn and the Epoch ofReionization. Comments are provided on the effect of the hyperfine structure ofhydrogen on the rate of heating or cooling of the Intergalactic Medium. It isshown that heating of the still neutral hydrogen by the Cosmic MicrowaveBackground is negligible, with a characteristic heating time of 1e27 s/ (1+z)at redshift z.

Intergalactic heating by Lyman-alpha photons including hyperfine structure corrections