  The exquisite properties of Rydberg levels make them particularly appealing
for emerging quantum technologies. The lifetime of low-angular-momentum
laser-accessible levels is however limited to a few $100\,\mu\mathrm{s}$ by
optical transitions and microwave blackbody radiation (BBR) induced transfers
at room temperature. A considerable improvement would be obtained with the few
$10\,\mathrm{ms}$ lifetime of circular Rydberg levels in a cryogenic
environment reducing the BBR temperature. We demonstrate the preparation of
long-lived circular Rydberg levels of laser-cooled Rubidium atoms in a
cryostat. We observe a $3.7\,\mathrm{ms}$ lifetime for the circular level of
principal quantum number $n=52$. By monitoring the transfers between adjacent
circular levels, we estimate in situ the microwave BBR temperature to be
$(11\pm 2)\,\mathrm{K}$. The measured atomic coherence time
($270\,\mu\mathrm{s}$) is limited here only by technical magnetic field
fluctuations. This work opens interesting perspectives for quantum simulation
and sensing with cold circular Rydberg atoms.
