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Aims. Other stars are known to have magnetic cycles similar to that of theSun. The relationship between these activity variations and the observedradial-velocity is still not satisfactorily understood. In this first paper ouraim is to assess the long-term activity variations in the low end of the mainsequence, having in mind a planetary search perspective. Methods. We used asample of 30 M0-M5.5 stars from the HARPS M-dwarf planet search program with amedian timespan of observations of 5.2 years. We computed chromosphericactivity indicators based on the Ca II H and K, H{\alpha}, He I D3, and Na I D1and D2 lines. All data were binned in to average out undesired effects such asrotationally modulated atmospheric inhomogeneities. We searched for long-termvariability of each index and determined the correlations between them.Results. While the S_CaII, H{\alpha}, and Na I indices showed significantvariability for a fraction of our stellar sample (39%, 33%, and 37%,respectively), only 10% of our stars presented significant variability in theHe i index. We therefore conclude that this index is a poor activity indicatorat least for this type of stars. Although the H{\alpha} shows good correlationwith S_Ca II for the most active stars, the correlation is lost when theactivity level decreases. This result appears to indicate that the CaII-H{\alpha} correlation is dependent on the activity level of the star. The NaI lines correlate very well with the S_Ca II index for the stars with lowactivity levels we used, and are thus a good chromospheric activity proxy forearly-M dwarfs. We therefore strongly recommend the use of the Na I activityindex because the signal-to-noise ratio in the sodium lines spectral region isalways higher than for the calcium lines.

Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program I. Comparison of activity indices