And then you hear the sound, from somewhere very close. As abruptly as it starts, it stops again. Your mind races, trying to place it. It sounds like a lion. A moment later, another sound. Siamang gibbons this time. A Sumatran tiger howls. Hold on - what continent is this? Africa? Asia? Didn't I go to bed in Australia?
And then you remember where you are, and it all makes perfect sense.
Melbourne's Zoo is one of the world's oldest and most elegant. It is also an innovator, offering visitors the opportunity to camp out in the zoo grounds overnight. The program is called "Roar 'n' Snore".
"It is a unique experience," says Melbourne Zoo communications manager Judith Henke.
"People really appreciate the chance to see what the zoo is like at night. It is a bit eerie because we keep the zoo very dark at night.
"Roar 'n' Snore visitors go on an after dark zoo tour, and there are usually some special extra feeding times for them to observe. Guides use red spotlights to illuminate animals without frightening them. This is followed by a close-up experience in the Discovery and Learning centre. We have noticed a lot of people choosing it for birthdays, anniversaries, those celebratory occasions."
Just a couple of kilometres from the heart of Melbourne, Roar 'n' Snore guests wine, dine and sleep where mighty elephants once walked, in the disused elephant enclosure. (The elephants now reside in a state of the art Asian-themed area on the opposite side of the zoo.)
As for the noisiest neighbours, Ms Henke points the finger at either the gibbons or the lions. "We have four African male lions, all brothers from the same litter, and sometimes they decide on a roaring competition. It's sibling rivalry, we assume."
For details on this experience visit www.zoo.org.au