  Using zoom-simulations carried out with the adaptive mesh-refinement code
RAMSES with a dynamic range of up to $2^{27} \approx 1.34 \times 10^8$ we
investigate the accretion profiles around six stars embedded in different
environments inside a (40 pc)$^3$ giant molecular cloud, the role of mass
infall and disc instabilities on the accretion profile, and thus on the
luminosity of the forming protostar. Our results show that the environment in
which the protostar is embedded determines the overall accretion profile of the
protostar. Infall on to the circumstellar disc may trigger gravitational disc
instabilities in the disc at distances of around ~10 to ~50 au leading to rapid
transport of angular momentum and strong accretion bursts. These bursts
typically last for about ~10 to a ~100 yr, consistent with typical orbital
times at the location of the instability, and enhance the luminosity of the
protostar. Calculations with the stellar evolution code mesa show that the
accretion bursts induce significant changes in the protostellar proper- ties,
such as the stellar temperature and radius. We apply the obtained protostellar
properties to produce synthetic observables with RADMC3D and predict that
accretion bursts lead to ob- servable enhancements around 20 to 200 $\mu$m in
the spectral energy distribution of Class 0 type young stellar objects.
