Exposure to light (specifically light in the blue portion of the spectrum) suppresses your brain’s production of melatonin. This hormone helps to regulate your biological clock. If you are exposed to light before bedtime or at night, it can dysregulate your biological clock and keep your alertness levels higher than you want them to be.
The best way to minimize your exposure to blue light before is to obtain special glasses that block 99%+ of blue light. These are typically amber colored. If they are clear, it is likely that they are what are sometimes called "computer glasses" that may block about 40% of the blue light. That is not enough for the purposes of treating Delayed Sleep-Wake Phase Disorder. These glasses cost about $10 and can be obtained in the Corvallis Clinic Sleep Medicine Department on the 2nd floor of the Asbury Building. They can also be obtained online. Recommended models include: Uvex Skyper or Uvex S0360X (for wearing over eyeglasses). You should begin wearing your glasses at least 90 minutes prior to bedtime.
It is also a good idea to keep the ambient light low in the evening and use black out curtains in your bedroom.
Special software can be can be installed on electronic devices to limit blue light output (e.g., F.lux; Twilight). On smartphones there are often settings to reduce light output at night. Unfortunately, these methods do not eliminate enough of the blue light.