Summary of problems:Research scientists do not find Kettlewell's work invalid because he released moths in the daytime. This claim is found nowhere in the research literature.
Full discussion:Bernard Kettlewell did several experiments on peppered moths, to explore the factors driving their observed evolution from lighter to darker forms over a relatively short time period. The research most relevant to this claim are the mark-recapture experiments where moths were captured, marked, released onto trees and recaptured the following night.
In these experiments the moths were placed onto trunks and branches at dawn, not day time, and allowed to take up their own resting places, as described in Kettlewell's 1958 paper "The importance of the micro-environment to evolutionary trends in the Lepidoptera" (Entomologist, 91:214-224). This is exactly what the moths do naturally. In one experiment, as a control, Kettlewell released moths earlier, and allowed them to fly on to the trees themselves. The recapture patterns from this experiment were no different from the recapture patterns with the moths placed on branches and trunks (Kettlewell, 1956, "Further experiments on industrial melanism in Lepidoptera" Heredity, 10: 287-301).
As well, some of the moths that were released in the mark-recapture-experiments stayed out for two nights before being captured. That is, they had been flying free at night and had found their own location during the morning. The distribution of those moths that did freely choose their own resting places is no different from those that were placed on trunks and branches (as shown in Kettlewell, 1956, and in his 1955 paper "Selection experiments on industrial melanism in Lepidoptera," (Heredity, 9: 323-342).
While there were legitimate reasons why scientists did criticize Kettlewell’s experiments (including Bruce Grant's 1999 paper "Fine tuning the peppered moth paradigm," Evolution 53. 980-984 and Michael Majerus's 1998 Melanism: evolution in action, Oxford University Press, Oxford, chapters 5 and 6), none of these criticisms (density and resting place choice) involve the moths being sleepy or sluggish, and no serious experimenter suggested that Kettlewell’s results were invalid. Indeed, subsequent experiments to test these criticisms broadly confirmed Kettlewell’s results (again, see Grant, 1999, Majerus 1998, and Majerus' 2007 talk "The Peppered Moth: The Proof of Darwinian Evolution," given at the ESAB meeting in Uppsala on 23 August – also available as Powerpoint, as well as his 2009 paper "Industrial melanism in the peppered moth, Biston betularia: an excellent teaching example of Darwinian evolution in action," Evolution: Education and Outreach 2(1):63-74). Further details of these experiments are discussed in "Where Peppered Moths Rest," below.