Summary Sensory systems continuously adjust their function to match changes in the environment. Such adaptation produces large perceptual effects, and its pervasiveness makes it a key part of understanding cortical function generally [1-3]. In visual contrast adaptation, for example, brief exposure to vertical stripes can dramatically alter the apparent orientation and intensity of similarly oriented patterns (e.g.; [4-7]). However, many environmental changes are long lasting. How does the visual system adjust to such challenges? Most past work on contrast adaptation has adapted subjects for just a few minutes. Only a few studies have examined durations greater than 1 hr [8-12], and none have exceeded 1 day. Here, we measured perceptual effects of adaptation in humans who viewed a world lacking vertical information for 4 days continuously. As expected, adaptation increased in magnitude during the first day, but it then showed a drop in strength. The decrease in adaptation is surprising because the adapting environment remained constant, and in short-term work, adaptation always strengthens or at least is maintained under such conditions. It indicates that the classical effects of contrast adaptation, which arise largely in primary visual cortex [13-18], are not maintained after approximately 1 day. Results from day 2 through day 4 further showed that slower adaptive processes can overcome this limit. Because adaptation is generally beneficial overall, its limits argue that the brain is sensitive to costs that arise when the neural code changes [19, 20]. These costs may determine when and how cortex can alter its function.