Here are the clues to solve the problem and get the animals back into their own shelves so that they can get a good sleep...
1. The LION is on the middle shelf.
2. The GIRAFFE is on the top shelf.
3. The TIGER is on the bottom shelf.
4. The MONKEY is in between the TIGER and the GIRAFFE.
5. The ELEPHANT is in between the LION and the TIGER.