Early and late subjective night phases of the Drosophila pseudoobscura circadian rhythm require different energies of blue light for phase shifting.

Two phase points (18 C.T. and 19 C.T.) in the subjective night of the Drosophila pseudoobscura circadian rhythm of eclosion, representing the end of the first half of the night and the beginning of the second half respectively, were exposed to blue (442 nm) light pulses of varying energies. The 18 C.T. phase was less sensitive relative to the 19 C.T. phase which is nearly 10-times as sensitive. This difference is not a feature restricted to the two phase points studied since similar differences could be traced to rhythm attenuating stimuli offered slightly earlier or later than the subjective midnight. It is suggested that the differences in sensitivity to light displayed by the two halves of the night are inherent to the oscillator driving the rhythm.