It's natural to feel crestfallen when, in the middle of telling your girlfriend about football practice, you hear zzz's. Before deciding she finds you boring, consider the legitimate reasons for her impromptu naps. If she regularly falls asleep during the daylight without provocation, she may need to see a doctor.

Sleep Quotient

Even though adults can get by on 7 to 9 hours of sleep, teenagers need between 8 and 1/2 and 9 and 1/4 hours, according to the National Sleep Foundation's article, "How Much Sleep Do We Really Need?" If your girlfriend stays busy with academic and social commitments, she may find it difficult to get as much sleep as her body needs. Even when someone reports that she's feeling good, a person who routinely get by on less than the ideal amount of sleep usually suffers for it, sometimes to the point of falling asleep at inappropriate times.

The Comfort Zone

If your girlfriend often falls asleep while talking on the phone with you -- maybe, it's the environmental cues that are to blame. The National Sleep Foundation identifies several elements that can factor into how easily a person falls asleep, which include noise level, the temperature and how much light there is. Your girlfriend can keep alert while talking to you by brightening up the room, sitting up straight and making sure the room is warm enough, because temperature drops can also cause drooping eyelids.