My eldest cat enjoys the nightlife and never wants to stay in once darkness falls, except when it is pouring with rain. Unfortunately she had to have her tail removed after being shot and, despite having to wear a collar for two weeks (to prevent licking the wound), she would not settle in the house at night. After a few sleepless nights (us) during which she dragged at carpets, doors and anything else she could get to, our vet suggested we try Feliway.
The diffuser plugs into a normal socket and the emitted scent is designed to be soothing to cats (I could detect a very faint odour but nothing unpleasant or overpowering). They are designed to work one per room and with a door closed ideally - we allow our cats to go everywhere so put one in the room where our cat normally sleeps and two more in other parts of the house. She did seem to settle more subsequently and we were able to reduce medical sedation and pain relief as well (and also get more rest ourselves).
The Feliway diffuser worked for our cat and if you have a cat that needs to be calmed down post-operatively, I would certainly give this a try. The instructions also suggest it can be used to reduce stress at other times, such as when there are fireworks, but the unit needs to be plugged in for a few days before it becomes fully effective, so it needs to be bought in advance ideally.
This is the refill for the Diffuser, which you need to purchase separately (one unit comes with the diffuser).