You're fine to use it in a cot. I used lights on a timer rather than the clock but the method is basically the same. Initially I set the light to come on not long after his usual WU time so that he didn't have long to wait. If he was quiet/chatting I left him to it until lights on, if he was calling for me or crying I went to him and treated like a night wake, kept the room dark, whispered, soothing words, "Everything is okay. It's night time go to sleep" kind of thing but kept him in the room as I would at say 2am (some times I verbally reassured from outside the room, sometimes I went to him, it depended on his mood and need). When the lights came on I changed my face and tone and did a bright "Good Morning!" and lifted him. I tried to get out of the room whilst still 'night' if I could so that my return was 'morning'.
Maybe you could begin at 5.30 sun up and once WU is consistently at 5.30 or later you can move the sun to 5.45 for a few days then 6am etc. You want there to be success so I wouldn't go for 7am right away as it is too long for a LO to wait.