Research indicates the people who suffer from depression also tend to experience insomnia. however, it is unclear whether the depression causes insomnia or the lack of sleep causes depression. this is an example of the

the directionality problem

Explanation:

In directionality problems, there are two variables and when these two variables are co-related then it becomes impossible to decide which variable is the cause of the particular problem and which variable is the effect of the problem.

So here there are two variables depression and insomnia but scientists can not determine that out of insomnia and depression which is the cause of the problem and which is the effect of the problem so it is unclear that whether the depression causes insomnia or the lack of sleep causes depression. Therefore this is an example of directionality problem .