There is just something about sunlight that is problematic for individuals with this type of fear, fear of dawn or daylight, which is called Eosophobia and is also otherwise known as Phengophobia.
Causes of this phobia are believed to be stemmed from other types of fears. For instance, strong social fears which make them prefer to be in darkness because it allows the individual to limit encounters with other people. Fear of open spaces is another associated fear. Fear of bright lights is another, where daylight is an unbearable time except cloudy days. Another somewhat irrational cause of this fear is if a person questions whether he is a vampire or not thus he gets scared of daylight because he fears that he might be turned to dust when exposed to the sun. This can be a sign of an underlying psychosis or delusion thus further investigation is warranted.
One of the marks of eosophobia is having a sense of dread during daytime. These individuals hide under heavy drapes to stay out of sunlight. Symptoms of eosophobia includes difficulty of breathing, tremors, sweating, weeping, increased or irregular heartbeat, anxiety, and feeling faint if exposed to sunlight.
Therapy sessions are needed to learn about your fear and help understand if there may be other fears that are linked to the fear you are experiencing.