Somatization is a common symptom of depression. Somatization is also related to sleep problem including insomnia.Depression is the one of the most common cuase of insomnia. Therefore, it would be needed to investigate the interaction between depression, insomnia and somatization.

To investigate the independent effects of major depressive disorder (MDD) and insomnia on somatization.

To compare somatization of primary insomnia, MDD with insomnia, MDD without insomnia, and normal controls.

A total of 181 participants without serious medical problem were recruited. Subjects were divided into 4 groups based on the SCID-IV and ICD-10 insomnia criteria:

1. 1) normal controls,

2. 2) primary insomnia,

3. 3) MDD without insomnia, and

4. 4) MDD with insomnia.

The somatization subscores of the SCL-90-R were completed by participants.

There were significant between-group differences in somatization (F=25.30, p< 0.001). MDD with insomnia showed higher somatization compared to normal controls (p< 0.001), primary insomnia (p=0.01), or MDD without insomnia (p< 0.001). Primary insomnia had higher somatization than normal controls (p< 0.01), while there was no significant difference between MDD without insomnia and normal controls. Presence of insomnia predicted higher somatization (beta=0.44, p< 0.001), while there was only non-significant association between MDD and somatization (beta=0.14, p=0.08).

In the current study, insomnia was associated with somatization independently from major depression. Subjects with primary insomnia showed higher somatization. Within MDD patients, presence of insomnia was related to higher somatization. Our finding suggests that insomnia may partly mediate the relationship between depression and somatization.