Causes of Drowsiness
Drowsiness stems from three major categories: primary sleep disorders (especially obstructive sleep apnea and narcolepsy), medications (particularly sedating drugs like antihistamines, benzodiazepines, and certain antidepressants), and underlying medical/psychiatric conditions (including hypothyroidism, neurological diseases, and depression). 1, 2, 3
Primary Sleep Disorders
Obstructive sleep apnea (OSA) is the most common pathological cause of daytime drowsiness, affecting approximately 24-32% of adults, characterized by repeated upper airway obstruction causing oxygen desaturation and sleep fragmentation. 2, 3 This is particularly significant because it is frequently undiagnosed yet highly treatable.
Narcolepsy presents with excessive daytime sleepiness, cataplexy (sudden muscle weakness triggered by emotion), hypnagogic hallucinations, sleep paralysis, and disrupted nocturnal sleep, with prevalence of 0.05%. 1, 2 Diagnosis requires overnight polysomnography followed by Multiple Sleep Latency Test (MSLT) showing mean sleep latency ≤8 minutes with ≥2 sleep-onset REM periods. 1
Idiopathic hypersomnia causes excessive sleepiness despite sleeping >10 hours, with profound sleep inertia (difficulty awakening) and unrefreshing sleep. 1, 2 Distinguished from narcolepsy by MSLT showing mean sleep latency ≤8 minutes but <2 sleep-onset REM periods. 1
Medication-Induced Drowsiness
First-generation antihistamines like diphenhydramine cause sedation in 10-25% of users by crossing the blood-brain barrier and blocking central histaminergic receptors. 4, 5 The FDA label explicitly warns of "marked drowsiness" and advises avoiding alcohol, sedatives, and tranquilizers which increase drowsiness. 4
Mirtazapine causes somnolence in 54% of patients (versus 18% with placebo), making it the most common adverse effect leading to discontinuation at 10.4%. 6 This sedation is dose-dependent and occurs through antihistaminergic effects.
Multiple medication classes contribute to drowsiness including benzodiazepines, opioids, SSRIs/SNRIs, antipsychotics, beta-blockers, and certain antihypertensives. 7, 8, 9 The American Academy of Sleep Medicine specifically identifies SSRIs, beta-blockers, pulmonary medications (theophylline, albuterol), and narcotic analgesics as contributors. 8
Medical and Neurological Conditions
Neurological diseases including Parkinson's disease, post-traumatic brain injury, stroke, multiple sclerosis, and early Alzheimer's disease commonly present with drowsiness as an early manifestation. 1, 2 Early Alzheimer's can present with excessive sleepiness before other cognitive symptoms become prominent. 1
Hypothyroidism causes hypersomnia and should be screened with thyroid function tests in all patients presenting with drowsiness. 1, 2 This is a readily treatable cause that is frequently overlooked.
Heart failure contributes to nocturia and sleep fragmentation leading to daytime drowsiness, warranting electrocardiogram and brain natriuretic peptide testing. 1
Psychiatric Conditions
Depression and anxiety disorders are associated with drowsiness in 50-75% of cases, requiring bidirectional evaluation as sleep complaints may herald onset or exacerbation of mood disorders. 7, 8, 2 Depression screening is mandatory as it commonly presents with drowsiness rather than mood complaints, especially in older adults. 1
Insufficient Sleep and Lifestyle Factors
Sleep deprivation is the most common cause of drowsiness, affecting an estimated 20% of the population. 3 Poor sleep hygiene including irregular sleep schedules, excessive screen time before bed, and uncomfortable sleep environments contribute significantly. 2
Diagnostic Approach
Begin with detailed sleep history assessing onset, frequency, duration of drowsiness, and associated symptoms like snoring, witnessed apneas, cataplexy, or restless legs. 2 Use the Epworth Sleepiness Scale to quantify drowsiness severity. 7, 2
Laboratory evaluation should include thyroid function tests, complete blood count, serum chemistry, ferritin level (if restless legs suspected), and brain natriuretic peptide (if heart failure suspected). 1, 2
Overnight polysomnography followed by MSLT is necessary to diagnose central hypersomnias and rule out other sleep disorders. 1, 2 Brain MRI is recommended to identify structural neurological causes, particularly in middle-aged and older adults. 1
Critical Pitfalls to Avoid
Do not assume all drowsiness is OSA-related; central hypersomnias and medical conditions are common and require different management. 1 The American Academy of Sleep Medicine emphasizes that true sleepiness (tendency to fall asleep involuntarily) is uncommon in chronic insomnia and suggests alternative sleep disorders. 8
Multiple comorbidities often coexist, particularly in older adults with polypharmacy, and treating one condition may not fully resolve drowsiness if others remain unaddressed. 1, 2 Review all medications systematically as polypharmacy with multiple sedating agents creates additive effects. 8
Distinguish drowsiness (tendency to fall asleep) from fatigue (low energy, tiredness, weariness), as fatigue is the expected consequence of insomnia while drowsiness suggests alternative sleep disorders. 8 This distinction is critical for appropriate diagnostic workup.
Do not overlook hypersomnia as an early sign of neurodegenerative disease in middle-aged and older adults, warranting thorough neurological evaluation. 1