The purpose of this study is to examine a possible link between the time of day of exposure to a traumatic event and the risk of developing post-traumatic stress disorder. Clinical and biological correlates will also be examined from this perspective.

Inclusion Criteria:

1. Persons 21-65 years old, who have been exposed to an event meeting the following DSM-IV criterion for Acute Stress Disorder:

   Criteria A.1 and A.2 At least one of Criteria B1-5A.1

2. Who provide written, informed consent to participate in the study.

Exclusion Criteria:

1. Physical injury that would contraindicate participation or interfere with a subject's ability to give informed consent or cooperate with the screening or collection of initial measures. Examples include severe burn injury, life-threatening medical or surgical condition, condition requiring surgical intervention under general anesthesia, as indicated by Abbreviated Injury Scale (AIS), or by clinical judgment;
2. Head injury involving confusion, loss of consciousness, or amnesia;
3. Medical conditions such as extreme obesity, psoriasis, herpes, Cushing's syndrome, current infectious disease, current viral disease, tuberculosis, unstable diabetes or hypertension, myasthenia gravis, and heart failure. Persons taking medications that can interfere with the HPA axis (e.g.,steroids, betablockers,indomethacin) will be excluded;
4. Weight below 45 or above 120 kg.
5. Pregnancy (in suggestive cases, a pregnancy test will be performed);
6. Traumatic exposure that reflects ongoing victimization (e.g., domestic violence) to which the subject is likely to be re-exposed during the study period.
7. Overt psychopathology, intoxication, or under the influence of substances.
8. Evidence or history of schizophrenia, bipolar, other psychotic condition;
9. Prior history of PTSD;
10. Current or past history of dementia, amnesia, or other cognitive disorder predating trauma exposure;
11. Assessed serious suicide risk. -

Men/Women, age 21-65, who have arrived at the Chaim Sheba Medical Center ER within six hours of exposure to a traumatic event.