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It is possible to sleep in many places around in Bratislava. You can try to sleep at the railway station but there is a chance that the guards will ask you either to leave or to show them a ticket by which you are going to travel so just tell them that you are going to Vienna tommorow but you haven't bought the ticket yet and you will buy it yesterdaytomorrow. That should work. You can also try sleeping in the Sad Janka Kráľa which is located on the south side of the Danube river. It's a quite big park so that you should be able to sleep there without any problems. Another option is sleeping in the Pečianky les. Which is just a bit west from the Sad Janka Kráľa and is a forest where is a lot of place to hide and you can camp there without being noticed or disturbed.