Bell tower of First United Methodist Church in Chatham, Cape Cod, Massachusetts, at dawn (6:26 a.m. on 23 June 1986; solar elevation: 12°35.4′ above horizon; natural illuminance levels: 12,200 lux ambient and 6,300 lux in the line of gaze). Exposure to artificial light of comparable intensity can rapidly reset the human circadian pacemaker by about 6 hours. See page 667. [Charles A. Czeisler, Neuroendocrinology Laboratory, Brigham and Women's Hospital, Harvard Medical School, Boston, MA 02115]

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