a, Circadian rhythm in electrical activity of a representative OB neuron. Insets show 10 superimposed action potentials recorded at different times over 10 days to illustrate the reliable discrimination of the activity of a single OB cell. b, Chi-squared periodogram analysis of the firing pattern from the cell in a shows a dominant frequency at 22.7 h. c, Circadian firing pattern of an SCN neuron recorded for 16 days on a different multielectrode array. Insets show small action potentials, typical of SCN neurons. The period estimate for this cell, shown in d, was 29.2 h.