  We perform a statistical analysis of the temporal structure of long
Gamma-Ray-Bursts (GRBs). First we consider a sample of bursts in which a long
quiescent time is present. Comparing the pre-quiescence with the
post-quiescence emission we show that they display similar temporal structures,
hardness ratios and emitted powers, but, on the average, the post-quiescence
emission is roughly twice as long as the pre-quiescence emission. We then
consider a sample of long and bright GRBs. We show that the duration of each
emission period is compatible with the duration of an active period computed in
various inner engine models. At the contrary, if the inner engine is assumed to
be always active, i.e. also during the quiescent times, in several cases the
total duration of the burst largely exceeds the theoretical durations. Our
analysis therefore does not support the interpretation of long quiescent times
in terms of stochastic modulation of a continuous wind. Instead the quiescent
times can be interpreted as dormancy periods of the inner engine. Before and
after a dormancy period the inner engine produces similar emissions.
