Between 2013 and 2016 researchers fitted common swifts with miniature data loggers recording acceleration and light. The records showed that outside the breeding season the birds were flying almost continuously, with several individuals showing no signs of settling for the entire non-breeding period of about ten months.
The design permits it. Swifts feed on aerial plankton, drink by skimming, and mate on the wing. Their very short legs and long wings suit clinging to vertical surfaces rather than perching or walking, which is why a grounded swift struggles to take off from flat ground.
Sleep is the least settled part. Swifts ascend to considerable heights at dusk and are thought to sleep in flight, possibly with one hemisphere at a time as some other birds do, but direct brain recordings from wild swifts have not been made. The often-repeated claim that a grounded swift cannot take off at all is an overstatement.