Dissipative dynamical system

Question

Given a time dependent dynamical system on $\mathbb{R}^n$ of the kind $$ \dot{z}(t) = X(z(t),t), $$ if there exists a function $f:\mathbb{R}^n\times\mathbb{R}\rightarrow\mathbb{R}$ such that $$ \mathcal{L}_Xf = \nabla_zf(z,t)\cdot X(z,t)\leq 0\;\;\forall t\in\mathbb{R}, $$ can we say the system to be dissipative?