Electrical Nerve Stimulation During Sleep for Memory Consolidation

The purpose of this study is to investigate the effects of transcutaneous electrical nerve stimulation (eTNS) applied during sleep on memory consolidation and brain-body physiological coupling. Healthy college students will participate in four overnight sleep sessions in a standard sleep laboratory. Before going to sleep, participants will learn and complete two cognitive tasks: a motor sequence tapping task to assess procedural memory, and an N-back task to assess working memory. During the night, participants will receive either 2 Hz eTNS, 120 Hz eTNS, or a sham (placebo) stimulation on their forehead in a randomized order. Throughout the night, researchers will record their sleep using polysomnography (PSG), along with continuous monitoring of heart rate (ECG) and breathing (respiration). Upon waking the next morning, participants will be re-tested on both memory tasks. The primary objective is to determine whether eTNS during sleep can improve behavioral performance on procedural and working memory tasks overnight. Secondary objectives include analyzing the stimulation's effects on specific sleep brain wave patterns (such as slow oscillations and sleep spindles) and exploring the coupling mechanisms between brain activity, heart rhythms (Heartbeat Evoked Potentials), and respiration during sleep.

Inclusion Criteria: 1. Healthy young adults (e.g., college students), aged generally between 18 and 30 years. 2. Right-handed. 3. Body Mass Index (BMI) ≤ 30 kg/m\^2. 4. Capable of fully comprehending the experimental procedures and voluntarily signing the informed consent form. Exclusion Criteria: 1. History or current presence of neurological disorders, psychiatric disorders, or brain damage. 2. History or current presence of cardiovascular diseases (e.g., arrhythmias or abnormal nocturnal heart rate). 3. Presence of depressive or anxiety symptoms (assessed by Patient Health Questionnaire-9 \[PHQ-9\] score \> 9, or Generalized Anxiety Disorder-7 \[GAD-7\] score \> 9). 4. Presence of sleep disturbances or poor sleep quality (assessed by Insomnia Severity Index \[ISI\] score \> 8, or Pittsburgh Sleep Quality Index \[PSQI\] score \> 10). 5. Any medication use within the past six months that could affect stress responses, cognitive abilities, or sleep architecture. 6. History of smoking or alcohol abuse. 7. Known skin allergies to conductive paste, medical tape, or silicone electrodes. 8. Female participants who are in their menstrual period during the scheduled experimental sessions (sessions must be strategically scheduled to avoid this period). 9. Inability or refusal to abstain from caffeine, tea, and alcohol for at least 24 hours prior to and during the experimental sessions.