Applying techniques in neurophysiology to the study of circadian systems, Meijer pioneered the use of in vitro and in vivo electrophysiological recordings to characterize the neural basis of circadian light responses in the mammalian SCN. Meijer's early studies in rodents used direct electrical recordings to map the prevalence and properties of visual SCN neurons in response to retinal light exposure. Meijer went on to characterize the baseline and light-induced activity of the mammalian SCN through long-term recordings in freely moving rats, which established the ability of the SCN to produce circadian rhythms in neural activity in vivo.
