The term "peak" always refers to insulin action, while "nadir" always refers to blood glucose levels.
At the time the insulin is at peak, it's exerting the maximum effect on blood glucose levels.
This produces the nadir, or lowest point of blood glucose levels-both happen at the same time. When the insulin is peaking (working hardest), it's bringing the blood glucose levels down to their lowest point, or nadir. When insulin levels are high, blood glucose levels are low and vice-versa. 
Looking at time activity profiles for insulin, they illustrate when and how the insulin is working. What's high or low on an insulin time activity graph usually means the opposite when considering the effect it has on blood glucose levels. When the insulin is shown on them as peaking--working hardest--this is when the blood glucose levels are at their lowest point, or nadir.
Generally, blood glucose nadirs in the evening (post-evening insulin injection) are higher than those following the moring insulin injection. There is less activity in the evening as a rule; normal activity can help lower blood glucose.