The portable PSG machine can not be available any more.

The investigators are going to compare the sleep quality and quantity between dexmedetomidine group and midazolam group using 24 hour polysomnography in critically ill patients. And the investigators also compare the incidence of delirium between the two groups.

Sleep is very important for cognitive and immune function. However, patients in intensive care unit (ICU) have a very poor sleep.

Almost all ventilated patients in intensive care unit (ICU) take a sedative drug. In Korea, most commonly used sedative drugs are dexmedetomidine and midazolam.

The investigators are going to compare the sleep quality and quantity between dexmedetomidine group and midazolam group using 24 hour polysomnography. And the investigators also compare the incidence of delirium between the two groups.

Inclusion Criteria:

• sedation patients with dexmedetomidine or midazolam
• over 18 years old

Exclusion Criteria:

• Neurologic disease (stroke, seizure, dementia, hypoxic brain damage)
• Brain infection
• Alcoholics
• Major Depression Disorder, Schizophrenia, Anxiety disorder
• Hearing loss

Critical patients who take a sedative drugs (dexmedetomidine or midazolam).