  The time scales of variability in active M dwarfs can be related to their
various physical parameters. Thus, it is important to understand such
variability to decipher the physics of these objects. In this study, we have
performed the low resolution ($\sim$5.7\AA) spectroscopic monitoring of 83 M
dwarfs (M0-M6.5) to study the variability of H$\alpha$ / H$\beta$ emissions;
over the time scales from $\sim$0.7 to 2.3 hours with a cadence of $\sim$3-10
minutes. Data of a sample of another 43 late-type M dwarfs (M3.5-M8.5) from the
literature are also included to explore the entire spectral sequence. 53 of the
objects in our sample ($\sim$64\%) show statistically significant short-term
variability in H$\alpha$. We show that this variability in 38 of them are most
likely to be related to the flaring events. We find that the early M dwarfs are
less variable despite showing higher activity strengths
(L$_{H\alpha}$/L$_{bol}$ \& L$_{H\beta}$/L$_{bol}$), which saturates around
$\sim$10$^{-3.8}$ for M0-M4 types. Using archival photometric light curves from
TESS and Kepler/K2 missions, the derived chromospheric emission (\ha and \hb
emission) variability is then explored for any plausible systematics with
respect to their rotation phase. The variability indicators clearly show higher
variability in late-type M dwarfs (M5-M8.5) with shorter rotation periods ($<$2
days). For 44 sources, their age has been estimated using StarHorse project and
possible correlations with variability have been explored. The possible causes
and implications for these behaviors are discussed.
