What is absent in stage N1 sleep?

In stage N1 sleep, typically classified as the transition between wakefulness and sleep, sleep spindles are indeed absent. Sleep spindles are characterized by bursts of oscillatory brain activity that are predominantly found in stage N2 sleep. They are associated with memory consolidation and neural plasticity and do not appear until the individual has moved beyond the lightest stage of sleep.

In N1 sleep, the EEG pattern is primarily dominated by theta waves and a reduction in alpha activity as the person transitions to sleep. Delta waves, which are indicative of deeper sleep stages (primarily N3), are also not present during this light sleep phase. K complexes, which represent short bursts of brain activity often triggered by external stimuli, can sometimes occur in N1 but are more commonly associated with N2 sleep.

Thus, the absence of sleep spindles in stage N1 is a key characteristic of this sleep phase, distinguishing it from other deeper stages of sleep where such spindles are present.