Vasovagal syncope: This is the most common cause of syncope in young adults, often triggered by stress, fatigue, or prolonged standing. The patient's recent increase in stress and sleepiness could contribute to this diagnosis.

Dehydration or volume depletion: Although not explicitly mentioned, the patient's increased sleepiness could be related to dehydration or volume depletion, potentially leading to orthostatic hypotension and syncope.
Sleep disorders (e.g., obstructive sleep apnea, narcolepsy): The patient's excessive daytime sleepiness and need for naps could indicate an underlying sleep disorder, which might contribute to syncope.
Anxiety or panic disorder: The patient's recent increase in stress could be related to an anxiety or panic disorder, which might precipitate vasovagal syncope or contribute to hyperventilation and subsequent syncope.

Cardiac arrhythmias (e.g., long QT syndrome, Wolff-Parkinson-White syndrome): Although the patient has no family history of cardiac problems, these conditions can be asymptomatic until a life-threatening event occurs.
Cardiomyopathies (e.g., hypertrophic cardiomyopathy): Similarly, these conditions may not have a clear family history and can present with syncope as the initial symptom.
Aortic dissection or other vascular catastrophes: Although rare in young adults, these conditions can be fatal if not promptly diagnosed and treated.

Neurocardiogenic syncope with malignant vasovagal tone: A rare and more severe form of vasovagal syncope that can lead to prolonged asystole and even death.
Pheochromocytoma: A rare tumor that can cause episodic hypertension, tachycardia, and syncope.
Subclavian steal syndrome: A rare condition caused by stenosis or occlusion of the subclavian artery, leading to syncope and other symptoms.