As you alluded to @Silje heart rate (as we commonly call it in “sports-speak”) is very individual and also can vary for a large number of reasons. It can vary based on what you ate, how you slept, how warm/cold it is, how stressed you are, how well-rested, and even depending on what you are thinking about!
From a study on the topic: “HR measures are determined by multiple influencing factors, such as environmental (e.g., noise, light, temperature), physiological (e.g., cardiac morphology, plasma volume, autonomic activity), pathological (e.g., cardiovascular disease), psychological (e.g., mood, emotions, stress) conditions, and non-modifiable factors (e.g., age, sex, ethnicity), as well as lifestyle (e.g., fitness, sleep, medication, tobacco, alcohol) and determinants of physical activity (e.g., intensity, duration, modality, economy, body position)” (Sandercock et al., 2005; Buchheit, 2014; Fatisson et al., 2016; Sessa et al., 2018).
And the most important part:
"[…] a recent meta-analysis found that the direction of change [in heart rate] was the same for both increased and decreased performance (Bellenger et al., 2016). For example, vagal-related HRVrest increased parallel to both increased and decreased (aerobic) performance, representing either increased parasympathetic modulation or parasympathetic hyperactivity. Similarly, decreased HRex was observed in both concurrent performance increases (Buchheit, 2014) and overreaching-associated performance impairments (Bosquet et al., 2008). To date, the only promising approach for deciphering this dilemma lies in the contextualization of HR measures and the use of multivariate approaches (Bosquet et al., 2008; Lamberts, 2009; Plews et al., 2013; Buchheit, 2014; Bellenger et al., 2016; Capostagno et al., 2016; Bourdon et al., 2017; Hottenrott and Hoos, 2017; Thorpe et al., 2017; Coutts et al., 2018; Kellmann et al., 2018).
I other words, the way HR changes can be exactly the same wether you are fatigued or are performing better! Therefore, from a practical standpoint, you should not worry too much about HR and should not use it in isolation to determine if you are ready tp train or need to rest. You may use it in combination with other factors.
Joe Friel, in the latest edition of his book, The Cyclist’s Training Bible, has a helpful table listing the factors to take into account when determining your readiness to train, and gives a 1 or 2 score to each, so that if you have a total score of 7 or higher, you probably need to rest that day (p.178, Table 11.1):
Common morning warning indicators that should be taking into account:
- Appetite - very high/very low?
- Waking pulse- high?
- Enthusiasm vs. just wanting to stay in bed
- Motivation to train
- Overall feeling (fatigued? stressed?)
- Mood (unusually grumpy; easily angered?)
- Sleep (duration and quality)
- Muscles/joints (soreness?)
Heart rate variability (not the same as simple heart rate)
A fairly new product on the market that claims to help athletes track their readiness to train via Heart Rate Variability and sleep quality is Whoop. I have not used it, so cannot provide my opinion.
I hope this provides some background!!