Sequentially, set the alarm clock for a different time with ten — fifteen minute interval, and one day, having woken up at the right time, set it as your waking time and estimate the duration of sleep cycles.
The method is simple, yet it is used by almost all the oneironauts. Despite the trained perception and the conventional boundary between the sleeping and waking state, no one wants to miss the opportunity of having three to fifteen different dreams overnight.
When the «right» time is determined, the alarm-clock is set for three — four different time points.
You do not need this, at least at the beginning of practice.
But one “time tag” should be calculated, it is the approximate number of hours your body needs to rest, to recharge the batteries and to take the state of the REM sleep normally.