This study examines whether wakefulness during the biological night (2:00-4:00 AM) is associated with increased negative mood, impaired decision-making, and suicidal thoughts. Adults with a history of suicidal ideation in the past six months will complete laboratory and home-based assessments under varying levels of sleep pressure. Participants will be evaluated during late-night wakefulness and under conditions of both higher and lower sleep pressure. The goal of the study is to better understand the biological and behavioral mechanisms that may contribute to elevated suicide risk during nocturnal wakefulness.

Suicide risk is elevated during the biological night, particularly between 2:00 and 4:00 AM. Observational and epidemiologic data suggest that nocturnal wakefulness is associated with increased negative affect, impaired decision-making, and greater suicidal ideation. The "Mind After Midnight" hypothesis proposes that wakefulness during the biological night exposes individuals to a combination of circadian vulnerability and homeostatic sleep pressure that contributes to emotional and cognitive dysregulation.

This study will recruit approximately 90 adults with a history of suicidal ideation in the past six months. Participants will complete both home-based and laboratory-based procedures. Laboratory assessments will evaluate mood, hopelessness, suicidal ideation, impulsivity, and decision-making at different times of day, including 2:00-4:00 AM. To examine the role of homeostatic sleep pressure, participants will complete late-night assessments under two conditions: (1) high sleep pressure (remaining awake until 2:00 AM) and (2) low sleep pressure (sleeping and being awakened at 2:00 AM). Circadian factors will be assessed using behavioral measures and physiologic markers, including melatonin.

The goal of the study is to experimentally test whether nocturnal wakefulness contributes to suicide-associated cognitive and affective processes and to identify underlying chronobiological mechanisms.

Inclusion Criteria

• Age 18-55 years
• History of suicidal ideation within the past 6 months
• Habitual bedtime between 9:00 PM and 1:00 AM
• Habitual wake time between 6:00 AM and 9:00 AM
• Ability to provide informed consent

Exclusion Criteria:

• Current suicidal intent requiring immediate clinical intervention
• Diagnosis of a primary sleep disorder (e.g., untreated obstructive sleep apnea, narcolepsy)
• Bipolar disorder or psychotic disorder
• Substance use disorder within the past 3 months
• Use of medications that significantly affect sleep or circadian rhythms
• Night shift work or transmeridian travel within the past month
• Medical or neurological condition that would interfere with participation
• Pregnancy

This study is open-label. Participants and study personnel are aware of the assigned sleep pressure condition (high sleep pressure or low sleep pressure), as the experimental manipulation involves remaining awake or being awakened during the biological night.