I have a sort of similar goal, sleepyhead[bmndr], that ran into the same problem! My solution was to increase the slope, but award fractional points for being late, decreasing every minute. I’ve been tweaking the formula, but right now I’m using 0.5^{((x-1)/10)+1}, where x is the number of minutes I’m late, which works out to 0.5 points if I’m one minute late, 0.25 at 11 minutes late, and 0.13 at 21. I’m not totally sold on this particular formula, necessarily (any suggestions from readers would be welcome!), but the concept of decaying-every-minute fractional points has been successful at making me still be more motivated to go to bed at 00:06 instead of 00:26, even though I’ve missed my actual hard-line bedtime.