The main aim is to examine the influence of daylight and melatonin on the temporal variations of hemostatic factors with key roles in the hemostatic process and its regulation. Particular emphasis will be on exploring the role of melatonin in hemostasis by comparing subjects with tetraplegia with able-bodied control subjects.

Inclusion Criteria:

Intervention group:

• A diagnosis of tetraplegia for at least 2 years, and otherwise healthy.
• Must not use drugs permanently (except anti-spasmolytics).
• Must be able to swallow tablets.
• Must give written consent to participate.

Control group:

• Must be healthy.
• Must not use drugs permanently.
• Must give written consent to participate.

Exclusion Criteria:

Both groups:

• Those with acute or chronic co-morbidity.
• Unable to cooperate.
• Not provided written consent to participate.