  A graph is said to be interval colourable if it admits a proper
edge-colouring using palette $\mathbb{N}$ in which the set of colours incident
to each vertex is an interval. The interval colouring thickness of a graph $G$
is the minimum $k$ such that $G$ can be edge-decomposed into $k$ interval
colourable graphs. We show that $\theta(n)$, the maximum interval colouring
thickness of an $n$-vertex graph, satisfies $\theta(n)
=\Omega(\log(n)/\log\log(n))$ and $\theta(n)\leq n^{5/6+o(1)}$, which improves
on the trivial lower bound and an upper bound of the first author and Zheng. As
a corollary, we answer a question of Asratian, Casselgren, and Petrosyan and
disprove a conjecture of Borowiecka-Olszewska, Drgas-Burchardt, Javier-Nol, and
Zuazua. We also confirm a conjecture of the first author that any interval
colouring of an $n$-vertex planar graph uses at most $3n/2-2$ colours.
