A homeless man suffered superficial burns when his blankets were set on fire as he slept in a Sussex shelter.
The 46-year-old, who was sleeping on the seafront in Marine Parade, Worthing, woke early on Thursday to find his duvet smouldering.
He called police just after 0100 GMT and was taken to Worthing Hospital.
Police, who called the incident a "cowardly act", found a burnt toilet roll at the scene and say three teenage boys were seen nearby.
They want to hear from a cyclist seen heading from Marine Parade to the pier and then to the Promenade at the time.
Det Insp Trevor Bates added: "Luckily the man was wearing several layers of clothing and this no doubt prevented him from receiving serious burns."