  We consider Lyapunov exponents for flat bundles over hyperbolic curves
defined via parallel transport over the geodesic flow. We refine a lower bound
obtained by Eskin, Kontsevich, Moeller and Zorich showing that the sum of the
first k exponents is greater or equal than the sum of the degree of any rank k
holomorphic subbundle of the flat bundle and the asymptotic degree of its
equivariant developing map. We also show that this inequality is an equality if
the base curve is compact. We moreover relate the asymptotic degree to the
dynamical degree defined by Daniel and Deroin. We then use the previous results
to study properties of Lyapunov exponents on variations of Hodge structures and
on Shatz strata of the de Rham moduli space. In particular we show that the top
Lyapunov exponent function is unbounded on the maximal Shatz stratum, the oper
locus. In the final part of the work we specialize to the rank two case,
generalizing a result of Deroin and Dujardin about Lyapunov exponents of
holonomies of projective structures.
