By definition, in order to have a peak, you need to have a downside. Without it, the graph would be flat or the landscape just a rolling hill.
Too often, people have this image of the peak in their mind and may even consider the climb to get there, but they overlook the journey down. In the office environment, this may manifest itself as a letdown after the big push on a project or a void to fill following achievement of a quota. Personally, it could show up as a regression on health or fitness goals after running that marathon or reaching the desired weight. You could experience it as the post-holiday or post-graduation blues when the buzz ends and everything feels ordinary.
Keep climbing that mountain and pushing the graph upwards – and keep in mind that if you reach a peak, the only way to go (at least temporarily) is down.