Activation of suprachiasmatic nuclei and primary visual cortex depends upon time of day

<jats:title>Abstract</jats:title><jats:p>The human suprachiasmatic nucleus (SCN), the master biological clock, is a small (∼2 mm<jats:sup>3</jats:sup>) and deep structure located in the anterior hypothalamus. Previous methods do not allow <jats:italic>in vivo</jats:italic> study of the human SCN in a non‐invasive manner. Therefore, we explored blood oxygen level‐dependent (BOLD)‐functional magnetic resonance imaging (fMRI) with OFF–ON–OFF block‐designed visual stimuli to record the activities in the ‘SCN and peri SCN in the anterior hypothalamus’ (SCN<jats:sup><jats:bold>+</jats:bold></jats:sup>) and the primary visual area V1 using a 3T Siemens scanner and six normal subjects. We found that: (i) the BOLD‐fMRI response to light and the mean of percentage activation in the SCN<jats:sup><jats:bold>+</jats:bold></jats:sup> at midday was significantly less than that at night; and (ii) the number of activated voxels in most of the visual area V1 at midday was significantly higher than that at night. We conclude that BOLD‐fMRI responses to light in the SCN<jats:sup><jats:bold>+</jats:bold></jats:sup> and the V1 areas vary with time of day. This conclusion is consistent with: (i) the previously measured phase–response curve to light [<jats:italic>J. Physiol.</jats:italic>, <jats:bold>549.3</jats:bold> (2003) 945] for the SCN activity at critical intensity threshold; and (ii) the interaction of the melanopsin‐based signals with the rod‐cone signals at the ‘giant’ retinal ganglion cells [<jats:italic>Nature</jats:italic>, <jats:bold>433</jats:bold> (2005) 749] for the V1 activity.</jats:p>