  We generalise sheaf models of intuitionistic logic to univalent type theory
over a small category with a Grothendieck topology. We use in a crucial way
that we have constructive models of univalence, that can then be relativized to
any presheaf models, and these sheaf models are obtained by localisation for a
left exact modality. We provide first an abstract notion of descent data which
can be thought of as a higher version of the notion of prenucleus on frames,
from which can be generated a nucleus (left exact modality) by transfinite
iteration. We then provide several examples.
