Normal Variance in Sleep Needs: Some individuals naturally require more sleep than others, and sleeping 10 hours a night can be within the normal range for certain people, especially during periods of high physical or mental activity, without necessarily indicating an underlying pathology.

Depression: Although daytime sleepiness is not present, some forms of depression can lead to increased sleep duration (hypersomnia) without excessive daytime sleepiness.
Sleep Inertia: Waking up from a long sleep period can sometimes leave individuals feeling groggy and disoriented for a while, but this does not necessarily translate to daytime sleepiness throughout the day.
Recovery Sleep: After periods of sleep deprivation, the body may require longer sleep periods to recover, which can sometimes be misinterpreted as a need for more sleep than usual.

Hypothyroidism: Although less common, hypothyroidism can cause increased sleep needs without necessarily leading to daytime sleepiness. It's crucial to rule out this condition due to its potential long-term health implications if left untreated.
Sleep Disorders (e.g., Sleep Apnea, Insomnia): Certain sleep disorders can lead to increased sleep time without daytime sleepiness, especially if the disorder results in poor quality sleep that does not leave the individual feeling rested.

Idiopathic Hypersomnia: A rare neurological disorder characterized by excessive daytime sleepiness and prolonged nighttime sleep, but cases without daytime sleepiness can occur, making it a rare consideration.
Kleine-Levin Syndrome: A very rare condition that involves recurring periods of excessive daytime sleepiness and prolonged nighttime sleep, though it's more known for its episodic nature and other symptoms like confusion and automatic behavior.
Narcolepsy without Cataplexy: Although typically associated with excessive daytime sleepiness, some forms of narcolepsy might not present with the usual symptoms of daytime sleepiness, especially in early stages or in atypical cases.