Self-discipline--the ability to delay gratification--is a significant predictor of whether you will achieve your long-term goals. If you always choose instant gratification over delayed gratification(e.g. staying in your bed rather than going for a run), you will face severe consequences in the long-term (e.g. becoming immobile).
The importance of self-discipline is applicable in almost every aspect of your life. For example, a famous experiment at Stanford University in 1972 shows that your level of self-discipline can predict the level of your academic success better than your intelligence score.
In the experiment, often referred as the Marshmallow Test, a child was offered a choice between one small instant reward (one marshmallow) or two small rewards (two marshmallows) if they waited long enough. Then the child was left alone in the room for 15 minutes.
In their follow-up studies, they found that the children who waited longer tended to have better life outcomes (e.g. better academic scores, educational attainment and body mass index).
Although the result of this experiment remains highly controversial, it suggests that improved ability to delay gratification could lead to better life outcomes.
It's easy to choose instant gratification over delayed gratification if you're not conscious. When I was an undergrad student, I often chose to go out rather than working on my assignments. I chose doughnuts over salad. I chose to nap over going to the gym. I chose to spend on stupid stuff I didn't even need over saving.
But since I heard about this experiment, I decided to see every decision I had to make as the marshmallow test. Whenever I'm tempted to take instant gratification, I ask myself:
Do I really need this? Will this help me achieve my long-term goals?
And the answer is almost always NO.
This heuristic doesn't make you a self-discipline master, but it does remind you of the importance of focusing on your long-term goals.
Remember: you have to sacrifice the present to get the future you want.