Tonight just keep alternating them but make sure the mantra is personal to your experience or they can just become words with no meaning behind them. What I like to do is point out dream signs from the previous night that I missed and develope a short mantra that reaffirms that I won't miss it a again. The more specific the mantra is to your previous experience the better I think. That's why it must change day to day. Tonight try to time your meditation session I'm not a veteran with meditation but I have gone as long as 30mins. And felt like only 5 mins passed. Test your meditation thresh hold.