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.

This study utilizes a randomized, double-blind, sham-controlled, within-subject crossover design to evaluate the modulatory effects of transcutaneous electrical nerve stimulation (eTNS) during sleep on memory consolidation and cardiopulmonary-brain coupling.

Healthy young adults will be recruited and rigorously screened to exclude those with sleep disorders, neurological or psychiatric conditions, or recent use of medications affecting sleep or cognition. Each participant will complete four overnight sessions in a standard soundproof and lightproof sleep laboratory, separated by 5-7 days. The first night serves as an adaptation night with no stimulation or data collection. The subsequent three nights involve experimental recordings under varying stimulation conditions (2 Hz eTNS, 120 Hz eTNS, and sham stimulation) applied in a randomized order.

Experimental Procedure:

On experimental nights, participants will arrive at the laboratory at 21:00. Prior to sleep, participants will undergo a training and baseline testing phase for two distinct cognitive tasks: a procedural memory task (the classic sequential finger-tapping task) and a working memory task (the 2- to 5-back task). Following the pre-sleep tasks, participants will be fitted with polysomnography (PSG) equipment, electrocardiogram (ECG) sensors, and respiratory monitors. Lights-out is scheduled for 23:00. Upon awakening between 07:00 and 08:00 the following morning, participants will be re-tested on both the sequential finger-tapping task and the N-back task to assess overnight memory consolidation.

Stimulation Parameters:

During sleep, eTNS will be delivered via a three-channel current source stimulator connected to self-adhesive silicone electrodes placed above the left and right eyebrows. The stimulation consists of biphasic rectangular pulses with a 250 μs pulse width. For the active conditions (120 Hz or 2 Hz), the current is delivered in cycles of 30 seconds on and 30 seconds off. The current intensity will be titrated to a subjective score of 3-4 on a numerical rating scale (NRS), perceived as a mild tingling sensation that does not disrupt sleep. The sham condition delivers current only for the first two minutes (the first two cycles) to mimic the initial sensation of the active conditions. All stimulations automatically terminate after a maximum duration of 8 hours.

Data Collection and Analysis Framework:

Continuous PSG recording (including standard EEG, EOG, and EMG based on the international 10-20 system) will be synchronized with simultaneous ECG and respiration data. The study will primarily compare the pre- and post-sleep behavioral improvements across the three stimulation conditions. Furthermore, the neurophysiological data will be analyzed to extract sleep structural features, slow oscillation (SO) and sleep spindle metrics, and complex physiological interactions. Specific focus will be placed on SO-spindle coupling, SO-respiration coupling, and Heartbeat Evoked Potentials (HEP) to elucidate the underlying neural and autonomic mechanisms by which peripheral stimulation influences cognitive functions 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.