Some studies have shown that there is a correlation between the activation of the sympathetic nervous system and difficulty in falling asleep.
This study conducted at the University of Pennsylvania revealed the role of ANs (autonomic nervous system) imbalance in causing insomnia, which seems to play a more important role than circadian rhythm disorder. These findings are related to central nervous system hyperexcitability rather than hyposomnia, which is usually related to no change or decrease in cortisol secretion or circadian rhythm disorder.
This study tested the sleep patterns of 8 insomniacs and 8 normal sleepers, and the results showed that normal sleepers followed the expected progressive autonomic decline, and insomniacs showed persistent sympathetic hyperactivity. These data suggest that although normal sleepers follow the expected progressive autonomic decline, persistent sympathetic hyperactivation is detected in insomnia.
Another study showed that "good" sleepers who stimulated the sympathetic nervous system for a week had the same difficulties as insomniacs.
Therefore, most of the reasons for insomnia of insomniacs are because the nervous system is too excited when playing mobile phones late at night, and the nervous system can't calm down before going to bed.