Final answer:
Difficulty sleeping related to distinguishing night from day is associated with the pineal gland, which secretes the sleep-regulating hormone melatonin.
Step-by-step explanation:
The difficulty sleeping due to the body's inability to differentiate between night and day is primarily due to disturbances in the production of melatonin, a hormone which regulates the sleep-wake cycle. This hormone is secreted by the pineal gland, an endocrine gland located within the brain. Melatonin secretion is influenced by the level of light in the environment - it is stimulated by darkness and inhibited by light, consequently affecting sleep patterns.
Cells in the retina sense light and convey signals to the suprachiasmatic nucleus (SCN) in the hypothalamus. From there, information is transmitted via nerve fibers to the pineal gland, adjusting melatonin production accordingly. Therefore, the gland in question that is involved when the body cannot tell the difference between night and day is the c) Pineal gland.