Found this explanation of the different effects comparing work vs rest interval length ....
The Benefits of HIIT
A HIIT workout offers myriad benefits. First, HIIT provides a great workout for your legs. If you perform HIIT a few times per week, you probably donít even need to do strength training for your legs, unless you have a desire for bigger legs. More importantly, HIIT really ramps up fat burning. The intense intervals allow for the release of fatty acids into the bloodstream. Additionally, HIIT results in increased HGH levels. HGH is a hormone that burns fat while preserving muscle. Finally, and possibly most importantly, HIIT results in EPOC, an after-burn effect which causes you to burn calories for hours after your workout is completed.
The Drawbacks of HIIT
HIIT is not perfect in every way. The main drawback is that you canít perform this routine every day. Overtraining is a serious problem, especially if you perform strength training for your legs as well. If your muscles are tired, you are probably better served doing a slow paced steady state cardio routine on that particular day.
Sprint Interval Length
There are a few components of HIIT that you can vary. The first is the length of the sprint intervals. Shorter intervals of 15-30 seconds allow you to exert more effort during the sprints. This increased level of effort will result in a stronger release of HGH. Additionally, these shorter intervals will release more fatty acids into the bloodstream.
Longer intervals of greater than 30 seconds require more perceived effort. These result in a greater number of calories burned. Additionally, these longer intervals deplete glycogen levels (carbs) allowing your body to burn more fat after the completion of a workout.
The recovery length also impacts the effects of HIIT. This is the walking or jogging portion of the workout that allows your muscles time to recover. The length of recovery is relative to the sprint interval. If you sprint for 30 seconds and recovery for 30 seconds, the ratio is 1:1. If you sprint for 15 seconds and recover for 45 seconds, the ratio is 3:1.
The longer the recovery in relation to the sprint interval (2 or 3:1), the more effort you can exert in the next interval. This increased effort will again result in a stronger HGH release. Additionally, longer recovery reduces the risk of overtraining.
A short recovery relative to the sprint interval (1:1) results in lactic acid buildup, glycogen depletion, and a greater after-burn effect (EPOC). However, this can lead to a greater risk of overtraining