Genetic research into depression is one of the greatest challenges in psychopathology. Despite an inheritance of 30% -40%, research has yielded no replicable genes. Sleep problems independently contribute to the development of depression and this dissertation highlights the relevance of this relationship for one of the most studied genetic variations in depression: the serotonin transporter gene linked polymorphism (5-HTTLPR). The findings described show that 5-HTTLPR affects both the risk of sleeping problems and sleep-related changes in depressive symptoms and hormonal stress reactivity. Pharmacological enhancement of serotonin synthesis could potentially reduce this genetic variation in the susceptibility to sleeping problems.