Can watching tv before bed effect my sleep?

Yes. Sleep and dreaming can be greatly affected by your experiences throughout the day and specifically what you are doing right before you fall asleep. Watching a tense or horror filled tv show will affect your dreams and may lead to nightmare. A pleasant relaxing show may give you good dreams and good rest. Be selective with what you put in your head prior to failing asleep.
Yes it can. Watching tv, especially close to bedtime or worse in bed can make that natural unconscious connection with the bed and sleep less strong. This may result in longer times to fall asleep. New data suggests that lights and bright screens from tvs or even mobile devices can affect our circadian rhythms which are important for regular sleep schedules.