Fig. 1.

Change in illumination at the Earth’s surface with solar (positive) and lunar altitude (negative) above the horizon; typical illumination levels of artificial light at night (ALAN); and levels at which nighttime lighting has been observed to have biological effects [arrows; Sharma et al. (1997) (a), Zubidat et al. (2007) (b), Johnson (1979) (c), Stone et al. (2009) (d), Kuijper et al. (2008) (e), Riley et al. (2012) (f), Bedrosian et al. (2011) (g), Miller (2006) and Kramer and Birney (2001) (h), Falkenberg and Clarke (1998) and Clarke et al. (1996) (h), Santos et al. (2010) (j), Dauchy et al. (1997) and Cos et al. (2006) (k), Bachleitner et al. (2007) (l), Evans et al. (2007a) (m), Larsen and Pedersen (1982) and Dice (1945) (n), Dice (1945) (o–q); studies of levels at which nighttime lighting has biological effects are from Gaston et al. (2013, Table 3)]. Main figure modified from Beier (2006), with additional data from Kurtze (1974); Rich and Longcore (2006) and Gaston et al. (2013). SS Sunset, CT civil twilight, NT nautical twilight, AT astronomical twilight