While fixed action patterns and reflexes are clear examples of almost entirely instinctive behaviours, most behaviours are complex and consist of both instinctive and learned components. For instance, in imprinting a bird has a sensitive period during which it learns who its mother is. Konrad Lorenz famously had a goose imprint on his boots. Thereafter the goose would follow whomever wore the boots. The identity of the goose's mother was learned, but the goose's behavior towards the boots was instinctive. Similarly, sleeping in humans is instinctive, but how much and when one sleeps is clearly subject to environmental factors. Whether a behaviour is instinctive or learned is common subject of nature versus nurture debates.