[2] He went on to join King's College, Cambridge,[3] where he undertook his PhD research at the MRC Laboratory of Molecular Biology, under the supervision of Michael Hastings, on the subject of cAMP signalling in the suprachiasmatic nucleus of the hypothalamus (SCN).

[9] These observations were subsequently independently replicated [10][11][12] and extended[13][14][15] but were considered controversial at the time since transcriptional feedback repression had been thought essential for circadian rhythms in eukaryotes.

[18] The O'Neill group’s research is focused on the evolution and mechanisms of circadian timekeeping in eukaryotic cells,[19][20][21] and how biological clocks regulate cellular function to impact upon human health and disease.

[22][23] In a 2019 paper, published in the journal Cell, the group identified insulin as a primary signal synchronizing mammalian circadian rhythms with feeding time.

These findings predicted the striking 40% difference in the number of days that human burn injuries required to heal which they subsequently identified.