In today's lecture , we're going to examine how habits are formed and why they're so difficult to break . Psychologists estimate that around forty percent of our daily actions are performed out of habit . A habit is essentially a behavior that has become automatic through repetition in a stable context . Researchers often describe this process as a loop consisting of three distinct stages . First , there is a cue , which is a trigger that tells the brain to begin the behavior . This is followed by the routine itself , which can be physical , mental or emotional . The final stage is the reward , which helps the brain decide whether the loop is worth remembering . Over time , the brain begins to anticipate the reward as soon as it detects the cue . This anticipation is linked to the release of dopamine , a chemical associated with motivation . That explains why a habit can persist long after the original reward has lost its appeal . So how long does it actually take to establish a new habit ? A frequently quoted figure is twenty-one days , but this has little scientific basis . A study in London found that the average was closer to sixty-six days . Moreover , the time required varied enormously , from eighteen days to well over eight months . Simpler behaviors , such as drinking a glass of water at breakfast , became automatic much faster . The researchers also found that missing a single day had little effect on long-term success . This is encouraging for anyone who has abandoned a plan after one small failure . When it comes to breaking bad habits , relying on willpower alone tends to be ineffective . A more successful strategy is to keep the same cue and reward but replace the routine . Next week , we'll consider how these principles are applied in public health campaigns .