Emission reversal in the Ca II H line in the average spectra of (from the less to the more active) Gl 581 (M3, in magenta), Gl 667C (M1.5, in pink), Gl 433 (M1.5, in yellow), Gl 176 (M2.5, dark), and Gl 674 (M3, in blue). Within our 100 M dwarf sample, Gl 581 has one of the weakest Ca II emission and illustrates a very quiet M dwarf. Gl 674 and Gl 176 have much stronger emission and are both moderately active with an identified rotational period of respectively 35 and 39 days (Bonfils et al. 2007; Forveille et al. 2009).