The investigators proposed that gabapentin will increase slow-wave sleep in adult critically ill patients. Increasing slow-wave sleep will improve the patients' outcomes (shortening ICU length of stay, improving ventilator free days, increasing delirium free days) in critically ill patients, a university hospital, Thailand.

Up to 61% of critically ill patients have sleep deprivation in ICU. Sleep deprivation can cause delirium and lead to prolonged ICU length of stay and mechanical ventilator days. However, pharmacologic interventions to improve sleep quality and prevent sleep deprivation are poorly tested for efficacy and safety in ICU patients. There were reports of gabapentin increasing slow-wave sleep in healthy populations and insomnia patients with insignificant adverse events. Therefore, gabapentin might increase slow-wave sleep resulting in improving clinical outcomes in adult critically ill patients. The objectives of the study were to evaluate the efficacy, especially on slow-wave sleep, and safety of gabapentin in adult critically ill patients admitted to ICUs at Ramathibodi Hospital, a university hospital, Thailand.

Inclusion Criteria:

• Adults (18 years and older)
• Patients admitted to ICU not more than 24 hours at the time of randomization

Exclusion Criteria:

• Patients admitted to ICU less than 72 hours
• Patients receiving gabapentin within 14 days prior to randomization
• Patients receiving restorative drug (trazodone, mirtazapine, olanzapine, agomelatine, and pregabalin) within 7 days prior to randomization
• Patients having contraindications to gabapentin
• Patients receiving high dose vasopressors more than 1 hour during ICU admissions
• Patients presenting with severe respiratory failure (PF ratio < 100)
• Patients presenting with RASS < -2 at the time of randomization
• Patients having target RASS of <-2 during ICU admissions
• Terminal ill patients
• Pregnancy