Which factor is significant in determining an individual's ability to fall asleep?

Caffeine consumption is significant in determining an individual's ability to fall asleep because it is a central nervous system stimulant that can interfere with the sleep-wake cycle. When caffeine is consumed, it blocks the action of adenosine, a neurotransmitter that promotes sleepiness, thereby making it harder to fall asleep. The timing of caffeine intake is also crucial; consuming it later in the day can prolong alertness and reduce the overall quality of sleep.

While morning sunlight exposure, type of mattress, and daily exercise routine can all influence sleep quality, they do so in different ways and often do not have as immediate a direct impact on the ability to fall asleep as caffeine does. Morning sunlight exposure is beneficial for regulating circadian rhythms, a good mattress can enhance comfort, and regular exercise can contribute to better sleep patterns, but these aspects are less likely to act as barriers to falling asleep than caffeine consumption.